Hi zab,
Welcome to the Mattress Forum! 
I’m happy to assist you and provide some advice. While I can’t help with “what” to choose, I certainly can assist with the “how” part of things.
The first thing I would recommend is to “reset” how you are looking at mattresses and immediately stop relying upon reviews. While other people’s comments about the knowledge and service of a particular business can certainly be very helpful … I would always keep in mind that you are the only one that can feel what you feel on a mattress and I would be cautious about using anyone else’s suggestions, experiences or reviews on a specific mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences in general won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here), and should not be used as any sort of a reliable indicator about the quality or appropriateness of a mattress for your specific needs.
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, durability and PPP 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).
As part of your mattress research, 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. If you can’t find out this information, I would consider that a risky purchase and would recommend looking elsewhere.
For those that have a more restricted budget then post #4 here and the posts it links to also include many of the better lower budget online options I’m aware of as well. Some of these companies are members here of the site (listed in post #21 here) who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of options included in the choices there (some quite affordable) and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
Posts #1 and #2 in this topic also includes more information about the new "breed’ of “simplified choice” mattresses (aka “one choice fits all” or “universal comfort” or “bed in a box” or “disruptor” or “millennial” mattresses) that are available online as well that are typically in a budget range of between $600 and $1000 (queen size).
Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing. Post #3 here has more information and suggestions about heavier weights that is worth reading.
Subject to first confirming that any retailer or manufacturer on the list that you wish to visit is completely transparent ( see this article ) and to making sure that any mattress you are considering meets the quality/value guidelines here … some of the better options or possibilities I’m aware of in and around San Antonio and Austin is in post #2 here.
I don’t doubt that this is the case if your limited experience has been with less expensive innerspring mattresses using little and lower quality foams.
All memory foam will tend to be more difficult to reposition when sleeping, and one that is more “medium-firm” would be a memory foam mattress with less overall memory foam in the comfort layers, as all memory foam is quite plush and not very resilient.
The Nectar uses a 1" of 4 lb gel memory foam, a 3-inch 3.5 lb memory foam with a medical grade cooling and finally a 2.2 lb polyfoam core. They don’t list the thickness of the polyfoam core, or the thickness of the quilted gel memory foam panel or the density of that material. The polyfoam core uses a good density foam, but I would use some caution with the 3" of 3.5 lb. memory foam on top of the core, as that is a bit lower than I would normally recommend. They are a product assembled in the USA of components sourced from China and the US. They do not specific what is from where. I would caution against purchasing this item before knowing the complete specifications and component origins. I can’t speak to what you’re referencing with the “false marketing” comment.
I would be more cautious with Amerisleep, as they tend to use some lower quality materials than what I would normally suggest in the durability guidelines. Amerisleep, along with Astrabeds and Simplyrest, is part of onemallgroup, which specializes in viral marketing in partnership with Kim Tyrone Agapito.
You can use the search feature of the forum to pull up more information about the brands you listed, or you can read through the simplified choice thread here for a consolidated place of information about many of the one-size-fits-all mattresses, including the Leesa and Tuft and Needle.
Not at all, unless you give up researching and use reviews as your guidance. And as you are PhD candidate, I’m guessing doing a bit of reading and learning is right up your alley. 
You shouldn’t have a problem finding something using better density foams that will last you the shorter time frame you need.
Phoenix