The computer is fine to go online again after the drive has been removed and reformatted with a hammer.
That's why best way to handle this is with a live Linux USB stick and with no persistence.
Maybe somebody will convince the guys from Tails to add a copy of Ian Coleman BIP39 tool for this purpose
is it better to spread BTC on several addresses than leaving on one?
Not necessarily. But it can help you sleep better if you don't trust the numbers provided.
Something like the belief that "the lightning doesn't strike twice in the same spot"