Hi stephen714,
Welcome to the Mattress Forum! 
While I can certainly help with “how” to choose, the focus of this site is not to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress 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).
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
While again nobody can speak to how any specific mattress will “feel” for someone else or whether it will be a good “match” in terms of comfort, firmness, and PPP because this is too subjective and relative to different body types, sleeping positions, and individual preferences, sensitivities, and circumstances and you are the only one that can feel what you feel on a mattress … outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Regarding edge reinforcement, as you are using your mattress as a bit of a chair during the day and a place to perform work, personal testing will tend to be quite important. You’ll want to look for the interior specifications, specifically the design of the support core with the particular type of edge reinforcement system built into the perimeter. As you’re using your mattress like a chair at times, you might end up being best served with something using a stronger innerspring unit that has a high-density and firm polyfoam edge encasement tub built around the edge of that spring unit. Some of the newer pocketed spring unit using a few rows of firmer springs around the perimeter do provide a good edge system that prevents roll-off when sleeping upon the side of the mattress, but I’m not sure if they would feel as you desire for your specific application as a firm edge-seating surface.
You’d also want to make sure that whatever product you were choosing uses higher quality comfort materials (as linked to earlier in this reply), as you’re sitting up in your mattress and placing a very specific weight concentration on the mattress, using it more as a seating surface in that instance.
As for some of the companies you mentioned, unfortunately many of the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay, which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
For a better option to consider, the closest vetted site members to your area (within two hours or so) are:
Magic Sleeper
Shovlin Mattress Factory
Urban Natural Home Furnishings
They would certainly be worthy of at least a phone call to see if they have, or could custom manufacture, something that suits your specific needs.
You can also perform a forum search on Philadelphia (just click on the link) to see what stores have been discussed in your area previously, but I have discontinued the provision of listings of potential retailers in various geographic regions (unless they are already approved site members), because of the difficulty in maintaining such lists in a retail landscape that is constantly changing, and most importantly the confusion it was creating with the consumer members who incorrectly assumed that these businesses had indeed gone through the strict qualification process and were approved as members of The Mattress Underground. Such an assumption is unfair to both the consumers seeking assistance, as well as the very businesses and manufacturers who have indeed qualified the be members here of The Mattress Underground.
Aside from that, I would always confirm that any retailer or manufacturer that you wish to visit is completely transparent (see this article) and also make sure that any mattress that you are considering meets the quality/value guidelines here.
If you have any questions about specifications or certain products, feel free to post back on the forum and I’ll do my best to provide answers.
Hopefully that gives you a good start on your search.
Phoenix