The last column are the codes of the Reverse Holofoils of the set. But since the RHs, stamped cards, and promos of the EX era all have a single code each, you can basically ignore them. 
Overall, only the non-holo Commons will have three codes, and most (but not all) Uncommon and Energy cards will have two codes. Everything else (like non-holo Rares or Holofoil Rares, etc.) will have single codes. (PS: The same applies to German/French/Italian/Spanish/Portuguese, and Japanese Commons will have four instead of three different codes.)
Not that I know of. For decades the purpose of these codes have been mostly unknown, and still are for the most part. The only thing I know is that the Japanese codes can be used to get HD scans on an old mobile app called Pokémon Hiroba, according to this source. That still doesn’t explain why there are different codes, though. So even after all these years, the reason of the codes is still a mystery.
Yeah, teddyr had a similar conclusion recently here. Some codes are defintely easier to find than others, although when I was still collecting all codes for Pikachu in all languages, I haven’t really noticed too much of it tbh.
Glad to hear you’re able to find them without too much issues. 
Especially since I read you only buy Mint cards.
Yeah, there are only two in existence indeed, and Rusty (TCA Gaming you contacted) sold most of them years ago. I’m lucky to have bought his second Pikachu at the time, but this was way back in 2017 iirc. 
Most are in collectors hands these days, so finding a specific Matchprint like Charmander is indeed tough. And they’ll likely be more than multiple thousands of USD if they do eventually pop up for sale.
But it’s a prototype card, and therefore nice-to-have imo. It was never intended to be released.
Anyway, glad I could help. Best of luck finding the remaining 9 cards.
Greetz,
Quuador