So my take -
1) Yes if you spend Y to live and your SO brings X home from work and Y>X then you need to provide Y-X or whatever other sort of agreement you come to.
Taxes will be based on if you're married. If you are then you can still file jointly, your taxes a couple will significantly lower with you stopping work (I'm making many assumptions here the best thing to do is just check the tax tables as you near FIRE).
2) I would use SWR; your interest will fluctuate based on returns and the market, the concept of SWR is regardless of returns but has a certain amount of flexibility to it due to that SWR being adjusted to a personal CPI rather than returns or actual inflation.
2a) As above I don't recommend interest as being the benchmark for withdrawals.
2b) Can I rephrase your question since it doesn't really jive with 2)? - What stock/bond mix will provide a return which will allow me to adjust my SWR based off of a personal CPI? Take a look at
this topic which was covered last week. Go to the first link in it. Table 3 outlines some potential mixes which may help you come to a conclusion.
:)
*edit for url fail