Hoses? Cut and replace. If they're so brittle they won't move, break them, get a pick tool, shatter it all off and remove it. Buy replacement hose, or make one. Cheap, not worth saving, just move it along.

The taillight thing, Dremel. I would Dremel the head off with a cutter wheel (likely trying to cut a flat groove in it first, using a larger flat screwdriver first. Does it 95 times out of 100).

Learning your solvents. Sometimes vinegar is needed. Sometimes mineral spirits. Sometimes just wd40, sometimes you have to get really mean and use something so strong, you wonder if you have enough PPE. When you start to look for the nail polish remover, its probably getting serious.

Lots of things respond to heat. Heat it up, it comes out. There are cheap induction heaters for the head sof bolts on Amazon. That, or a torch. Just a bottle torch works--propane bottle top ones are fine.

Buy yourself some impact drivers. The ones you hit with a hammer, to get screws and bolts out. These things are often a LOT safer than using an air driven impact gun, because the slam DOWN with the hammer will jar threads and corrosion better than the 100 percent pure twist action in air impacts.

A cheap tap and die set. The harbor freight one for plastic and aluminum works ok, but, not for anything steel. Often, it's JUST to chase threads, so that your new or old hardware can go in and out like new again. Can pre-chase rust off some of the longer studs so they don't bind half way removed and make you want to yeet the car into the river.I use that fairly often. I'm also fine, with non critical parts, letting bolts soak in light weight motor oils, and cleaning with a show towel, but no solvents, before putting them in. They won't work out, but they also have some corrosion resistance I'm ended in them now, pretty much forever.