Oh golly, after all you’ve mentioned in your reply today, no wonder you’re filled with stress and worry. This situation with your teeth is certainly compounding things for you. I can empathize with you for the huge impact this is having on the quality of your life and health. Are you able to eat ok?

Well, the positive take away is that you still have your lower teeth except for 2 molars. If your periodontal disease is under control with regular dental cleanings and you maintain them at home with daily flossing and brushing, you should be able to keep those.
Your new dentist is prudent having you wear the occlusal guard to help keep your remaining teeth safe from stress grinding. It’s called bruxism. The guard will also help hold your jaw in alignment with what’s left of your remaining upper teeth.

The upper arch, having collapsed from the loss of teeth would make it difficult to have a good degree of holding power for a removable denture. As your current dentist mentioned, it would need to be held in place with dental adhesive such as poly grip. It would basically be for cosmetic purposes and very light eating as the grip wouldn’t be very strong.

It is entirely possible that you do have plenty of bone remaining to do the All on 4 technique as this was designed for situations like yours. The pins are put in at an angle in the back and are shorter, not requiring as much bone. They are also generally implanted with bone grafting material. But it is very costly.
This site I posted may be similar to the brochure you have. It gives some information about what’s involved and shows how the All on 4 implants works and some information
https://sharedentalcare.com/all-on-4-dental-implants/
Another consideration you might try is going to a university dental school. UM Kansas City has a dental program and it looks as though there are a couple other dentistry school in the state. They often have clinics where their dental students work with patients for a reduced fee. The dental students are supervised and overseen by their instructors.
https://dentistry.umkc.edu/
Do you think this could be a possibility for you?