1) you will get another derived key from that mnemonic and it can be used as BIP32 seed but it won't be standard and you'll have to remember the same iteration for the future recovery.

2) no because they will also have to guess your custom iteration count which increases the number of keys to brute force drastically and may even make it impossible

3) true but it would be reasonable to assume that if your mnemonic is leaked your iteration count can leak too.

4) i believe the current secure number for PBKDF2 is more than 10 million if you are going to do it once (which is the case when creating a wallet)