I’ve just setup 2 trezor safe 5’s. At this time I used a single phrase backup. However, will migrate to a multi-phrase backup at a later date. On reading, I see the single phrase backup will still be “active” for the same wallet (an hence you could destry the backup paper - even though it still exists).
My question, if i first create a multi-phrase backup (i.e not a single phrase). Will there be a single phrase backup word (theoretically exists) that can access the wallet (i.e. I would not know it - but it would exist)?
Not unless you entered the threshold number of shares from the multi-share backup into the Trezor later, and created a single-share backup from it using the upgrade to multi-share backup process in Trezor Suite.
technically, yes. all possible variants of your backup, incl. single-share (and also e.g. 16-of-16), exist mathematically.
Note that this does not affect security of the whole system. An attacker that doesn’t know your backup phrase would have to guess a 128 bit secret, which is not possible, regardless of how you choose to split or not to split it.