Part time yes, at the moment I'm not aware of any FO's doing it and only a couple of captains, you would need someone to jobshare with.
Your personal tax affairs are your own so as long as you don't break any laws pay tax where you like, non UK residents get some less benefits.
FO becomes SFO after er, ATPL and min 1 year in company I think, makes no difference to money.
Non typed pilots have about 5K less pay in year 1 than typed, after that its the same. Cadet entry have different pay scales for first 4 years.
For you last question - if you join now - how much do you like sauerkraut?