When should I do my next water change?
Since you just put in bio spira you might want to give it a couple days to attach itsself to something as its free floating when you first put it in.
How often should I change the water?
Normally once a week should be fine unless you feed more than 2 times a day or have a heavy fish load for the tank.
When should I start vacuuming the gravel?
After putting in the bio spira I would wait a few days then lightly vaccum the surface of the gravel to remove fish wastes. Also I would feed lightly while going through this kind of cycle.
When the cycling appears to be done (if the Bio-Spira fails completely), will I need to re-treat for ich when introducing a few new fish?
You should have a seperate tank for new fish and keep them there for a couple of weeks. If you do not do this you will always run a chance of introducing a nmew disease everytime you introduce a new fish.
When this is done, should I replace the canister filter innards for any reason, or wait until the manufacturer recommends changes?
I have not used a canister filter but I would not replace anything but the charcoal in anyfilter you can rinse the media in tank water then put it back in. Some one with more experience with a canister type filter will be able to help alittle more.
As far as treating with meds, salt and heat. Yes you can run into problems if your not careful because you are subjecting the fish to alot of stress while they are already under stress from the ICH. You could try the salt treatment and heat for a week and see how that goes. My own methods were done after researching a ton of information about Ich and the various treatments. The theroy behind no light is that the ICH cannot see if theres no light and it makes it harder for it to find the host. Also the fish are under less stress when there is some darkness and They move less and use less oxygen in darkness. You will need to keep up the aireation for oxygen when raising the temp.
I can feel for you with just starting this tank and having to go through the treament of ich. One good thing from all of this is that the bacteria like warmer temps and will grow faster so it will help things as far as the cycle goes.
Ok enough rambling. What would I do?? I would change water daily 25% while treating for ICH. I would keep the lights low. I would not feed the fish very much for the next few days. I would add salt double dose over 2 days meaning 1 tablespoon per 5 gallon the first day the another dose the second day. I would keep the temp at 84F. I would vaccum wastes from the bottom during water changes daily. I would make sure my water temp when changing is exactly the same.
Anyhow I really hope things turn around I hate to see problems happen to people