In my opinion the mudcrab merhcant and creeper were easter eggs so it is the players choice weither they did or not. TES is about doing whatever you like so I don't think they perfer the player to take personally responciablity of abuse not force them into a box.
NPCs were supposed to have low max septim counts, A) because it was logically that the local fishing town weaponsmith doesn't have enough money to feed the whole of Vvardenfall.

To counterbalance having uber items around, if the player finds something really expensive and wants another really expensive item then they will either have to barter directly, sell many of them to buy it or find another one. Remember the winged twilight soul you can get at level 1, it would ruin gameplay if that single item suddenly granted you access to any item in the game, but if you are clever then you can use your mercentile skill to try to get as much as possible from selling it to one person.
Daedra are high level, daedra don't have expensive gear to high level, level 20 is supposed to be high level. The fact that people get to level 50 onwards is because they kept playing past the point of high level.
Training is quite expensive unless your high level and/or used an exploit.
So I'd be expecting 1. will be harder to find or not there. 2. Stay the same, perhaps a bit higher because as you've seen in TB they did realise people wanted high money traders and were exploiting the mudcrab anyway, but they did the right thing in making half of them have high mercentile skill (they would do if they earnt all that money) though I felt let down by the other half. In BM they toned it down again but still had high money traders, most likely because they were already in TB. 3. I'd say the high level point will be at a higher level. 4. Probably abit higher, I don't know, I never considered it too low.