1.) A razor saw in the little X-acto ur clone aluminum miter-box, and gap filling CA or white glue.

2.) Yes, sometimes. In any case a jig probably won't make it any better than careful board-building. The exception might be a dual-tapered wing. The kits should give you measurements for "lofting" jigs or blocks for these (unless you get a "builder's kit" that doesn't tell you squat). T-pins and a balsa or ceiling tile building board are sufficient. Get a couple plastic drafting/building 90º triangles (and maybe a handful of the House-of-Balsa alignment jigs.

3.) I have bult many planes using only white glue and epoxy, but since returning to building lately I use pre-assembled and wicked CA much more, especially for the wing ribs. It is just sooo much faster. But if the odor of the CA bothers you, as it does some folks, it can be eliminated. An accelerator (official or 50/50 household ammonia & water in a spray bottle) is a great help, but I have heard it can weaken joints. There are three "speeds" of CA and each has it's favored applications. I still get the occasional puzzlement where CA didn't bond so I scrutinize every joint.

My rule of thumb is that if it is a lap joint where both pieces are flat I use white glue & clapms or pins. If it involves an end grain I use CA. For the firewall and any spot that could contact raw fuel I use 15 minute epoxy (I never use 5 minute epoxy, ever).

I wouldn't hesitate to build a complete plane with white glue (I like Aileen's Craft Glue) and then coat the firewall and inside of the tank compartment with alcohol thinned epoxy or fuel proof dope. I built dozens of free-flight and control-line that way before CA was known.

If you narrow it down to a specific kit you are interested in ask here and you should get some great help.