Not at land borders though. These kiosks will probably only be available at airports. Thus for land arrivals, complete your card at your hotel before heading to the border or if you must, on your laptop or phone while at the border.
I asked about kiosks at land borders when I reentered Thailand by car the other day. The officer told me he has no idea but doesn't look like it. Most likely you'll need to use your laptop or smartphone to fill it out or borrow one. Best advice if traveling by land is to fill it out on your laptop while at your hotel the day you cross back to Thailand.
Not really. You can draw up some documents yourself and as long as someone is paying you and you have the 500,000 in the bank you've got a viable business as far as the consulate is concerned.
Your visa states "employment prohibited". The Thai authorities permit online work, but I wouldn't go around advertising you're working in Thailand as that can lead to misunderstandings and problems.
You'll only need to prove what you're doing if you go in for an extension, but you can always use an alternative such as a soft power activity if you're no longer employed.
I'd imagine a work contract you update and having the 500K in your account for at least 1 month would suffice though; you can easily update a work contract yourself even for a company you no longer work for.
Since we're talking work based abroad, Thai authorities really have no way of verifying anything.
Tax returns asked for at consulates. Can't see it being asked for at immigration in Thailand when entering the country. No way for immigration to verify its authenticity or to deal with lots of different languages.