Same here mate it was very minor the gasket blew a week later. Just keep an eye on it wipe it down and run the car and see if it stops at any point. Also would re-check the toque setting on the head bolts.
TD04L from an wrx or new age Subaru impreza plenty good second hand ones out there for manifold and down pipe would recommend a Wepr setup from Sacha flawless quality
In theory yes on minor wrapt surfaces LOL strange as it is the gasket will be going on my wall as an souvenir now haha. They defiantly are lessons learnt this time!
Could be your water temp sensor. That plays a part in controlling fuelling on cold start and on hot when they fail they usually cause misfire on cold starts and when hot
The straight edge is an old school technique it still works still see people using that method today and having no issues obviously its not %100 accurate but gives you an rough idea.the head has been skimed and checked recently was perfectly fine
To be honest if your running stock power and stock boost you wont be needing it you will simply gain lag and lose power unless your upping the boost abit then yeah