It’s nice to have but not a must. I like it for touch less washes where I foam the car with bilt hamber and rinse, without touching anything. Without CR I’d not risk it due to water spotting.
Replacing the resin every year kinda sucks but if you did the math to see how much regular washes at...
Test the TDS of your water before and after softener. The lower TDS you feed the CR, the longer the resin will last. But having CR before softener makes no sense as DI water doesn’t need to be softened and if you do, you’ll introduce solids that remained in the resin after a backwash.
BTW, I've applied SiC 4 hours after UK when I thought it was like Gliss and it worked fine as well. No ill effects like they say you can see if you take over an hour for the UK's 2nd coat.
Meguiar's #39 was great but they no longer make that. McKee's 37 has a product for that that works fine. Also a regular rubber eraser can be quite useful.
I much prefer flat pads. My CCS is my least favorite pad. It just seems to hold product in the dimples and after a few sessions it gets hotter than flat pads. Lake Country Thin Pros and Rupes yellow are my favorite.
If you do it right after polishing that is a possibility. That's why many like to wait a few hours and I opt to wait overnight. That gives the SiO2 in it time to cure and then Eraser just removes any leftover oils which may not be a lot if you polished with a machine.
I like to polish with Essence and then give it overnight to cure, then in the morning wipe with Eraser and coat. I don’t know if that’s the right way but it is a way that has worked well for me.