Hi Kendalizor,
I will try and answer each question, in the same order in which you posted:
1) In true pot limit, you only count what's in, but it's rarely played that way, so you would presume the $5 straddle + SB + BB both at $5, as the straddle is a new forced minimum, so that means $15 in the pot before any opens = a total 'raise pot' value of $25 total, ($5 + $20 raise)
2) I would allow the button to open for $20 more ($25 total), but that's purely to increase the pot and generate more rake (again not true pot limit)
3) Pre-flop yes, in cash games you would count the SB as the full forced bet.
The general aim of your cash game rules should be to allow as much money hits the middle as possible given the gamwtype, in order to maximise rake, playing by true pot limit will impair your pots substantially, so it is always better to be pretty 'flax' in your implementation of the 'pot limit'
Regards
Stuart