[General]
iHoursToRespawnCell = 240
iHoursToRespawnCellCleared = 720

...but I don't know how much they override default spawn times, if it includes mods or if they even work at all.

I'm also using a mod that reduces spawn. Will these scripts override that mod?

It would be fun to play through Skyrim once with no respawning at all to see what level I would end up after I've cleared it.

Those are the default values of 10 and 30 days. The only thing that can override those would be scripts changing the values during runtime (unlikely but possible) and mod-specific ini files that get loaded with the mods.

To be sure which values are active just use the console commands "GetGS iHoursToRespawnCell" and "GetGS iHourseToRespawnCellCleared".

Remove those lines from the ini file. If you want to adjust them, the best place is in the BASH patch. When building/recreating the patch, you can find those settings under tweak settings. This is where I set the game time multiplier to 12 (instead of the default 20).