NO. Better to get a DTV or another visa type you qualify for. A tourist visa or exemption should ideally only be used for multiple short stays well under the limit or for one stay up to the limit plus extension
Presumably, an applicant would wait until they're asked for more documentation before leaving the country, so what you say isn't usually a concern. Only a small handful of consulates perform interviews.
Most foreigners living in Thailand hold work permits. Therefore, if this functionality was limited in the past it affected only tourists, retirees and others who don't work here.
No, you don't do a 90 day report if you're leaving within 90 days anyway. You could probably get away with not doing a 90 day report even if you leave a little later (since you have until 7 days after the 90 day period to file one) so as long as you leave and come back, you're fine. It's once you need to do something at immigration that a 90 day report may be needed.
Ah, so you used an agent! That's why it went smoothly. I'm not saying Ranong is problematic, but come on any sort of visa that isn't an extension of stay and they'll grill you first before letting you leave (if you don't use an agent).
This happened to me last time on a non-O multiple entry visa and they tried to ask for 20K (I had almost that much on me, but that was to pay for my expenses in Myanmar, not for Thailand). Besides, credit cards are not accepted in southern Myanmar anymore, it's cash only, thanks to the political situation.
I talked my way out of the 20K cash requirement and since they let me leave after I explained what I was going there for, I had zero issues or questions asked on re-entry.
I will probably use a pier specific for the islands next time. Although I went on an island tour last year, I used the Saphan Pla pier to depart and come back, as I was staying in Kawthaung and did the tour to/from Kawthaung, rather than beginning and ending in Ranong, like many Thais do.
However, I think if you're using the services of one of the two companies that run those tours (there may be others as well) you are entitled to use their pier and immigration facilities.