no, schreenshots are not accepted. They want scanned bank statements for 1-6 months depending on what embassy you apply through.
And yes, you pay when you apply. It is a fee for processing the visa application (they do the process, no matter if the application is approved or denied).
If you stay 2-3 days each time, probably 20+ times... if you try to live in Thailand on visa exempt, just doing border bounces, you will get stopped sooner or later. There are no rule/law that regulates this. Also there are no specific for a calender year and no reset at new year. The IO check the combined travel history in your passport and if it looks like you're trying to live in Thailand without a visa, you will eventually be denied entry. If your travel history shows that you are a genuine tourist, there's no need to worry or count entries.
you should have stopped with "Purpose of visit: supporting docs that align with your purpose of stay." Then it would describe any DTV-application. (Employment and business licence is not for any of the soft power options.)
why would he have picked "to visit...". No, just as he have written in another comment, he chose the domestic partnership status since he doesn't consider themselfs as single.
And they DO ask for marital status on the e-visa platform when you apply for a regular tourist visa.
Not for proof of it, but for the information (similar to being asked to fill in Mr/Mrs or Male/Female in other forms).
I am not sure where that information is from, but that doesn't really matter. Of course it is not a requirement to be married or single or whatever, to obtain a tourist visa! However on the e-visa platform it is mandatory to fill in marital status (chose from a dropbox) even when applying for a tourist visa (just as you have to add date of birth, address etc). It is not a requirment to obtain a visa, but requiered information.
If you don't believe me, just try the e-visa platform yourself (just as I did).
I think that you are wrong about this. There are no tourist visa such as "Tourism Family". As for purpose of stay there are 6 options to choose from and they are very clear, see picture. (The only option that mention the word family is phrased "To visit family residing in Thailand" and I doubt that they chose that purpose.)
Even when choosing "Tourism/leisure activities" you are asked for marital status and I think this is where the OP made a mistake... (see other comments).