Hi DesertDuck,
Welcome to the Mattress Forum! … and I’m glad you found us 
I’ll do my best to answer your questions although some of them are more subjective or about preferences which are much more difficult to quantify relative to any specific person.
It certainly makes sense that your mattress would be too firm without the softer comfort layer … especially for side sleeping.
I’m not sure if you have a box spring with springs inside it or a foundation with a rigid support surface.
An mattress with a latex support core will generally do best with a firm, flat, and evenly supportive support surface underneath it that has minimal to no flex under the mattress and for larger sizes with at least one center support beam that has good support to the floor to prevent any sagging in the middle of the mattress. The components (either a bedframe and foundation or a platform bed) need to be strong and durable enough to support the weight of the mattress and the people sleeping on it without some of the parts bending, sagging, shifting, or breaking with extended use. The support surface under the mattress (which may be a solid surface, slats, or a steel or wire grid) should have enough surface area to prevent the mattress from sagging through any gaps or spaces in the support surface over time but ideally still allow some airflow under the mattress. If the support system under the mattress has a slatted surface then I would suggest that the gaps between any slats are no more than about 3" (with 1 x 3 slats) although less than that would be better yet.
You can see my comments about a solid support surface under a mattress (such as plywood or MDF) in post #10 here. While it would be “supportive” enough and in most cases they would probably be fine … they can be an additional risk factor for the formation or mold or mildew … particularly if there are other risk factors involved that could contribute to the formation or mold or mildew in a mattress.
Latex and memory foam are very different materials with very different properties but the choice between them is more of a preference and budget choice than a “better/worse” choice. There is more about some of the differences between memory foam and latex in post #2 here but the best way to know which type of materials or mattresses you tend to prefer in general would be based on your own careful testing and/or personal experience with each material in a range of different firmness levels. Strictly in terms of firmness (and not “feel” or resilience or the other properties of a foam layer) a latex layer in the same thickness and with an ILD somewhere in the teens would probably be closest but it will still be very different from memory foam. You will sink in more deeply into any softer foam layer than a firmer layer of the same material.
I don’t know how any differences in the design of your DIY mattress would affect your lower back. The first “rule” of mattress (or component) shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
Most people would notice a difference yes. A thicker layer of the same material in the same ILD would generally feel softer than a thinner layer of the same material and will isolate you more from the feel and firmness of the layers underneath it.
He sold blended Talalay latex made by Latex International (now called Talalay Global).
The better online sources I’m aware of for different DIY materials and components are listed in post #4 here.
4 lb or higher memory foam would normally be durable enough for those that aren’t in higher weight ranges (more than the lower 200’s or so) in which case I would lean towards 5 lb or higher memory foam. I would also keep in mind that there are many different formulations of memory foam that have different firmness ranges and different properties (such as slower or faster response, more or less breathable, more or less temperature sensitive etc) depending on the chemical formulation of the memory foam. There is more about the different properties that can be formulated into of different types of memory foam in post #9 here and in post #8 here.
Thanks for the kind comments … I appreciate it.
Phoenix