Must I repeat the prayers in which I gave the salutation before the Imam's second salutation, based on the madhhab I followed at that time? Does the prayer become invalid according to Imam Malik if I follow two different opinions in one issue? Is it permissible for me, as a Maliki, to adopt the views of other scholars in some matters, given my struggle with waswas (obsessive whispering)?

The worshipper must not say the tasleem before the Imam. If the worshipper says the first tasleem after the Imam's first tasleem and before his second tasleem, then his action is permissible and his prayer is valid, even according to the Hanbalis. The most correct view is that suffice it to one tasleem validates the prayer, and no re-offering is required from one who emulated a madhhab in an action of the prayer and then it became clear to him that it invalidates the prayer according to another madhhab.