If it's a blister, it's air. If it can keep in air, it can hold out water. He's a diy'er who wants a simple fix, for a roof that is, in the main, fine.
Over egging the pudding doesn't help. Does it?
If you are keeping loft area, you can insulate across the rafters as usual. 220mm of rockwall last time i looked.
3x2 won't take a concrete tile if 2ft spacing. You'll need to go back with slate or eternit type tiles.
Party wall not required.
It needs a gasket on the inspection plate, 5 minutes work. Make sure upstairs is not having a dump when its fixed.
Could also be a combination stack taking rainwater also.
Britmet or Metrotile go down to 5 degrees.
http://www.britmet.co.uk/lightweight-tile-effect-sheets.asp
Felt under with counter batten/membrane/slate is acceptable....but I wouldn't do it on my house.