Using a fence as the 'other half' of an antenna (groundplane) will work, it's not that unusual at all. Does it make for a good groundplane? Not particularly, chain-link fence parts ave very seldom ever 'solidly' connected to each other, post, fence segments, etc. But it will work. How about for scanning purposes? Not so good there. Not because of how much 'groundplane' you have, but how much height you don't have.
There are 'catches' though! If you are going to use that fence as part of a transmitting antenna, you just put your antenna as close to your neighbor's (and your) house as that fence runs. The whole neighborhood surrounded by that fence? Oh boy! You'll make everybody mad at you about RFI. That ought'a be fun, don't you think? (That's what I did with the fence around my yard, and neighborhood, as part of a station ground system. Found out real quick that it isn't the smartest idea in the world.)
That "take it down, trim it, put it back" thingy is the normal way of tuning an antenna. If you only have to do it once or twice, you are very very lucky. Get used to it. That's not just 'home-made' antennas, but all of them, they all require tuning, there are no "one size fit's all" situations antennas. That's sales 'hype'.
Can you put more than one antenna on the same fence? Sure, why not. Got more than one antenna on a car? Same thing just different way of looking at it.
Need to extent your feed line? Okay, do it. It's done quite often with no particular problems. Barrel connectors work. Sealing a connection like that if it's exposed to the weather isn't a bad idea. Probably several dozen ways of going about it, pick one.
Will a fence as a ground help with transmitting or receiving? Only in that it is a substitute for putting in a good 'dirt' groundplane, AKA radial system. There's no particular benefit to it, other than it's already there, sort of. If that's all you can manage right now, then do it! Nothing says you can't do it some other way later.
- 'Doc