The mastic will re soften as you need it too , I had to get a new lense from Zmanalex and that took a week to sort so don't worry about that. Don't bother with the oven just use a heat gun carefully, concentrating on the back of the headlight (the black plastic bit).
The silicone I used was exterior auto stuff so I guess the same as you have in mind mate.
I've got a used , slightly melted headlight you are welcome to practice on but your miles away from me! If you need any help let me know and I'll do my best
Edit: use the sealant (silicone) after its all back together as an extra outer seal. Don't add it to the original mastic that may not work!