Impossible to answer without knowing many different factors such as whether you are old enough to qualify for a retirement visa, or whether you can find an employer willing to get you a work permit and associated visa, or whether you are willing to pay for an elite visa, plus many other factors. The basic answer is that if you aren't somewhat wealthy and can qualify for either the new long stay visas aimed at the wealthy, or willing to pay the large costs associated with an elite visa, and aren't old enough to qualify for a retirement visa, or aren't willing to marry a Thai, then you can't plan to stay a long time on repeated tourist visas.
The major Thai banks publish their exchange rates online. Just add the typical 220 baht ATM withdrawal fee that is added regardless of amount withdrawn.
Strange why everyone is saying that people don't know Tylenol here. Tylenol brand is available at every 7-11 and Big C and Lotus and pretty much everywhere that sells paracetamol. It comes in a white strip of 10 caplets or in the normal bottles. As for the PM, they don't sell the version of Tylenol with Diphenhydramine in Thailand. You'll have to separately take Tylenol and antihistamines.
The Thailand Pass application takes 2 minutes to complete and approval for most people comes within hours, and starting 1 June, will become instantaneous. If you are capable of booking a hotel or plane tickets online, you certainly should be capable of filling out the Thailand Pass application.
The agency is offering to pay someone off the stamp you for a 30 day extension that you don't qualify for. Sounds perfectly ethical and not at all shady.
There are insurance plans linked to in the insurance section of the official Thailand Pass website tp.consular.go.th if you want to compare. There are cheaper options than the price you indicated. And of course all the plans linked on the Thailand Pass website qualify for entry.
"is there enough to see in Bangkok" is really a ridiculous question. Is there enough to see in London? In Tokyo? In New York? Surely you know Bangkok is one of the major global tourist destinations. I would understand if you were asking about Samui or Bristol or Denver, but asking about it for Bangkok is a bit bizarre.
This is a known issue that many can't find their application when checking status. If the hotel saw it to approve it, then it is definitely in the system so no need to worry.