1) Forget about the effect of CO2 on pH - it's temporary, and it's the other factors that affect pH that actually have a significant impact on living organisms. But, yes, ease into it, because a sudden large CO2 spike could have some effect on fish. I'd suggest increasing it daily over a week. I say this hypocritically, of course - I've always just set them up and gone full steam with no ill effects on fish, but others have had some issues. So be gentle.
2) No, don't worry about ever repeating the process, a few days of no CO2, and then back to normal, won't bother anything.
3) A few days without CO2 is fine. To be safe, you could dial down (or turn off) the lighting for that period, but it's probably not necessary. Last time I took apart my CO2 rig on a Thursday to tweak it, didn't finish before bed, had to leave town after work on Friday, and didn't set it back up until Monday evening. I had the lights off for the weekend, and ended up turning the lights and CO2 on when I went to bed Monday night, and then off when I woke up, and back to the original cycle (3pm to 11pm) that afternoon. No effects to speak of.
4) It wasn't really a question, but a backup tank is overkill. A backup regulator might be worthwhile, and I have several of those.