A 90 day report is simply you reporting that you've been in Thailand for 90 consecutive day. If you're not in Thailand, then you haven't been in Thailand for 90 consecutive days. You start counting again from day 1 when you return, but you won't be able to do it online then. You'll have to go in person as the first report after entering Thailand must always be in person.
STOP spreading misinformation. This is ONLY for Laos. You pay online everywhere else. In Vietnam you pay online. In Cambodia you pay online. In Malaysia you pay online. It's only Laos.
Technically no, if there is no stamp in your passport with a consideration date, then your permission to stay ends on the date in your passport as normal, because there is nothing in your passport giving you additional time. I would probably go back to Krabi on your last day if you cannot get ahold of them.
The thing he's referencing as must be printed is specific to Laos. After you apply for the visa online, you must wait for them to send you a confirmation with barcode. You print that and take it to the embassy or consulate where you applied between certain morning hours to submit payment for your visa application.
Same criteria possibly, but not the same procedure. The embassy and consulate in Laos is the only place where they will send a confirmation that you print and take to the embassy to pay. Everywhere else accepts online payment for the visa.
You should edit your post to include the specific Embassy or consulate that you used, whether it's Vientiene or Savannakhet.