HiHamb0ne10.
Welcome to our Mattress Forum! 
We’re happy to be of assistance! You can start a new topic any time in our expert members forums. Additionally, you can also append (as you did) your comment/question to an existing topic that closely matches your topic of interest (please see the sticky post here which explains this.)
Sorry that Purple mattress is not working out for you …I completely understand your desire to get the next one right. On the positive side, you can assimilate the experience you’ve had with your current mattress and pinpoint the reasons why this was not a good match for you and incorporate this understanding into your future research for a good fit mattress. It is also a great plus that you have the “freedom” to choose something that is suitable for you and know that it wouldn’t disrupt or affect the sleep of your gf.
Create-a-king products are good options to fill the gap between two separate twin beds and to secure the mattresses together for an even surface across the entire bed. How effective they are will vary with each product . The wide adjustable belt that fastens and wraps around the sides of both mattresses is ensuring that the mattresses do not separate even during more robust moving on the mattress. They can use foam fillers or a sheepskin, or other filling options to close the gap between the beds, but how effective they are at providing an even surface would again be different from product to product. Another option to connect the twins would be a quilted pillow top cover or mattress pad with a wedge that fills the space between two mattresses and corner straps to keep the beds together.
Of course, If your gf prefers a different comfort/support configuration, then you also have the option of a split king mattress. How much you would feel the split would also depend on the specifics of the mattress and on how flush the top surfaces fit together. With all the above options, you could also use a single topper over both sides (preferably with the gap filled in) which will further dampen the gap effect. You can get various types of “converters” which fill in the crack in the middle and attach the two halves together such as here and here and here and here
Even though you wake up on your back it seems that you are primarily a side sleeper (or … “forced” back in to it by your snores and gf ) it is best to aim for a side sleeper comfort in your mattress. From your description it looks like you already figured out that a key element when sleeping upon your side is to make sure that you are not sinking in too deeply with an excessive lateral curvature. This article speaks to this in a bit more detail. Side sleepers generally need a bit more pressure point relief on the surface to accommodate the wider dimensional variances between the shoulders and the hips and the waist. There’s a bit more about different sleeping positions here .
This is one of the most common questions on the forum (ie. which mattress type or firmness level would be better for me?) There is some fairly extensive general information in the mattresses section of the site (particularly in the sections about sleeping style, preferences, and statistics along with putting the layers together and the page on tips and tricks here) that can give you some general concepts, guidelines, and insights about the effects of different body types, sleeping styles, and mattress designs for different people but this is only generic and not specific to any particular person and may be more complex than you really need to know.
The mattress shopping tutorial here has the basic information, steps, and guidelines including suggestions about how to test a mattresses for for what I call PPP (Posture and alignment, Pressure relief, and Personal preferences) that can simplify your search and help you make the best possible choices. Perhaps most importantly it can also help you avoid the worst ones (like the major manufacturers or any manufacturer or brand that doesn’t disclose the quality of the materials in their mattresses and most mass market stores that have little knowledge about the mattresses they sell).
There are far too many variables and unknowns (body types, sleeping styles, preferences, physiology, age, health conditions, sensitivities, special circumstances etc) to use any formula, specs (either yours or a mattress), or “theory at a distance” based on basic height/weight/sleeping position information or any other person’s experience to choose a suitable mattress that can possibly be more accurate than your own careful and objective testing on local mattresses (using the testing guidelines in the tutorial post) that you can use to decide on a mattress that is “best” for you in terms of PPP (whether it is for a local purchase or as a guideline for an online purchase)
So I’m always happy to help the forum members here with “how” to choose, to act as a "fact check, or to help them find better manufacturers or retailers that have better quality and value but “what” to choose in terms of either a mattress or the balance between comfort/pressure relief and support/alignment that may be suitable for a specific person is always up to the only person who can feel what you feel on a mattress and best left to personal testing and your own experiences or more detailed discussions with the more knowledgeable people at the retailers or manufacturers that sell a mattress you are considering.
I think that it would be fair to say that you have more than one reason to eliminate mattresses that contain memory foam as sex on memory foam takes more “effort” because it doesn’t provide any of the bounce or “energy” return that most people are used to. You are certainly not alone in preferring more resilient materials (either in comfort layers or in a support core) that are less motion restricting in a mattress and either reducing or in some cases even eliminating mattresses that contain memory foam comfort layers which are much less resilient than other types of materials and components such as latex, polyfoam, or microcoils.
As far as motion isolation…memory foam is the best at motion isolation but latex and pocket coils are good for this as well (see post #18 here). Having said that … all the layers and components in a mattress will have some effect on the feel and performance of all the other layers and the mattress “as a whole” so the best way to test for motion isolation is your own careful testing on a mattress using the testing guidelines in the tutorial post (with both of you on the mattress) because a mattress that may be “motion isolating enough” for one couple may not be as suitable for another couple that is more sensitive. A foundation under your mattress would certainly contribute to having better motion isolation but it would depend on the type and force of the movement and on the thickness of your mattress as well (a thicker mattress would isolate you more from the springs). Traditional metal bed frames are also culprits in allowing people to feel more motion when moving atop a mattress.
Overall I would think it’s reasonable to say and most manufacturers would agree that latex is “good” with motion separation, especially the slower heavier types of movement such as turning over or bouncing or getting out of bed and are an improvement over other materials (except memory foam) but again this depends on the type of movement or vibration and the specific construction of the mattress, the ticking and quilting materials, and the materials or components above and below the latex and even the bedding above the latex as well.
I am not sure if you are aware of this but the stomach sleeping position is the worst posture for your lumbar region, regardless of core strength or tonus of your musculature. Everyone needs good deep support to assist with alignment and your muscles relax a bit when sleeping at night, so maintenance of alignment falls mostly to the mattress itself.
One step at a time (with not skipping any of the steps) is the most effective way to find “your perfect mattress” and has by far the highest chances of success … and of course once you’ve had the chance to read through the information provided and narrow down your research to a few finalists I or any of the Expert members of the site would be happy to help.
Phoenix