Nope. You’ll need to be in the country that you apply at. For sure it’s an online system so how would they know? They’ll ask for proof that you are in the country and will need to see stuff like tickets from that country to Thailand.
Could you “fake” your location? Possibly. Would you get away with it? Who knows.
You can get a non O in a nearby country, but you’ll need to check which one you’re going to use. Some are switching over to the evisa system as of the 8th October.
You can still go there to apply but it’s an online process. Because the change is so new we don’t have details of turnaround times that they will take.
Bulgarians now get visa exempt entry for 60 days. You should have no issue returning after 2 days in Cambodia, but as it’s at the discretion of the individual immigration officer no one could guarantee that.
That’s going to be a hard one to answer. Going back a few years even signing a cheque as a business owner was deemed “working” and therefore required a work permit and relevant visa.
These days it’s a bit more relaxed as what’s construed as “working” but still a very gray area.
The tax implications are a whole different level of concern. I’m no expert on those. Many countries have a dual tax agreement with Thailand so you shouldn’t be taxed twice on money brought in but you’d need to consult with an accountant or tax expert on that.