I can only partly answer your questions as I'm not really up to date with the latest state of 3DS hax, to put it lightly.

I believe the log just shows the game the flashcard is pretending to be, so if your flashcard says it is "Spongebob Squarepants", the log will show Spongebob Squarepants, regardless of which game you are actually playing on the card. As such, I think it doesn't particularly matter whether you share data or not if you're afraid they would find out. That said, I always disable data sharing anyway because it doesn't serve any real purpose.

It's never 100% safe, as you maybe remember from many years ago (time sure flies) some DS flashcard teams bricked the devices of people who bought knockoff versions of their flashcards. I think the same thing happened, again several years ago now, with one of the 3DS flashcard teams bricking devices of people who got fake versions of the flashcard. These are really exceptions, though.

It used to be that the flashcard could stop working after any firmware update, but I have no idea if this is still the case or if some kind of solution has been found to make sure the cards theoretically work forever.

Taking a quick look I found that the SuperCard team is now making a 3DS-compatible flashcard (DSTWO Plus). I've always been a fan of their products, so if I had to make an uninformed purchase, I'd probably pick them. I would suggest you ask these questions on another, more active website though, as it's probably not a good idea to follow the advice of someone who hasn't seen a flashcard in years.

Doviđenja!