I am not aware of any shared experiences yet what consulates accepted as proof of financials by means of a guarantor. You will have to check with the consulate in your jurisdiction - or wait for shared experiences / reports to come in on that. You could (in theory!!!) also borrow the money, transfer it to your account and use that as proof. BUT: Some consulates anticipated that and are already asking for bank statements over a longer period of time.
Worst (highest) temperatures between April and June. Worst air quality between February and May. Most rain between June and September. Optimal conditions between October to January.
From how I know things Thailand, I would assume that exact requirements to qualify will vary significantly from consulate to consulate. So, yes, best to wait and see what consulate might be most lenient in that case. I also agree that the potential to misuse this open visa might lead to requirements being tightened up at some point. I am not a big fan of playing one (age) group against another. But, being 42, I am happy that finally not only 50+ years people have it easy to be staying in Thailand more long term now.
You don't jave to have a job or work for the DTV ... that is only one of its two tracks. You can also book a recreation / soft power activity to qualify (at some consulates minimum length of bookings apply). BUT, you would still need to proof at least 500k THB in savings. Not sure if the monst fesable way if without a job.
Might work, but likely not. Some consulates reported that at minimum 3 months of booked activity needed. Also, would need to present a confirmation with stamp / seal and signature.
The Thai trains coming and going to / from Thailand are on the one side of the station on separate tracks, the trains coming and going from Malaysia are on the other side of the station also on separate tracks. In between are both immigrations. Be aware that those immigrations only open when a corresponding train on the other side has arrived and waits there ready to leave. Thus, there can be some waiting time involved crossing forth and back.
You book the train to Padang Besar - not Padang Besar (Thai). The Padang Besar (Thai) is a separate stop before the border still on the Thai side. The Padang Besar regular stop brings you right to the train station where the immigration is and where you can change to the Malaysian trains. I have not done a border run there, but I have entered from Malaysia to Thailand there. No particular issues with immigration. Not sure if for a pure border run they might ask you to stay at least a night before returning (at some land borders that can happen at will). Maybe someone else has done a pure border run there and can chime in.
Note, you could also exit at the Pedang Besar (Thai) station and make your way to the alternative road land border crossing nearby as an alternative.
You cannot "renew" a Non-B visa. The visa itself is issued only once for single entry of 90 days. Then the stay in Thailand can be extended year by year for another year based off of being employed with a work permit. That yearly extension of stay requires quite some documents from both the applicant and the organization he / she is working for. The actual extension itself costs merely ~2,000 THB of fees at immigration.