Useful, but not essential. If you pull on the wires a lot, it's quite easy to cut your hands. Cheap fingerless gloves are OK. Don't forget it will probably be hot, so normal gloves may be too hot. Also pulling on steel cables is a fast way to wreck your expensive goretex mitts!
This link seems to have a fairly comprehensive description of the law. It does appear that some types of temporary camping are allowed. As usual with all matters related to planning, it's probably best to get professional advice. Also not clear if you may end up having to pay Business Rates.
Speaking as a farmer the reason that many landowners are not keen on allowing camping is that you would need to obtain planning permission for "Change of use" to allow it. Probably not a problem for allowing the occasional individual, but if you had a large group and one of the neighbours complained a whole host of bureaucratic nastiness would descend on you.
I just used my normal walking boots. It depends on how much you want to use the "Artificial aids" to make progress. For the ferrata I have done, I tried to use the metal rungs, etc as little as possible, which generally involved Mod to VDiff climbing. Some things would be desperate/impossible without using the ladders, etc. It's up to you what you choose to use.
Climbs and descents can be long, so make sure whatever you wear is comfortable.
Don't know about Garda, but Selva and Cortina have lots of places you can hire gear.
I just carry an ordinary climbing sack (OMM Jirishanca) and a Platypus.
Many companies on the French side will offer carnets for driving in Switzerland as standard. Given that it's not that easy to avoid some Swiss motorway when you leave the airport, it's worth having one.
Avoid EuropCar. When I hired a car from them in Corsica a couple of years ago, they expected you to return the car in a fully valeted condition. Failure to do that would result in an extortionate "cleaning" charge.
Also be sure to read the CDW carefully. Common exclusions are damage to wheels and the underside of the car. Some exclude just about any sort of damage, making them effectively usless.a