You can't reserve land in the frontier yet, but certainly if you build a base out there you can still have your residence plot.
You can build as many bases as you want, as long as you don't build them too close to other bases, in the wilderness. In the waste and the wild there's a chance to get griefed though, even though it's illegal. You could make a base far out in the wilderness on one server and stay in town on an other server, just so you don't have to keep going back and forth.
Yes. You can have a base in every world on every server if you want. ATM, wilderness is unprotected. But griefing rules apply, anyone altering your build in any way can be banned. But stuff happens. It is recommended you travel very far out into the wild to make a base, far enough griefers will not find you, and you are not building near/on/under other peoples bases. (There are hidden bases all over the place). If you want to be able to 'protect' your base in the future, when Aikar gets to that, you must have it over 5k from any outpost, and over 3k from any other players build. So again, build wilderness base way way far out in the wild.
I don't think it's illegal to fix if someone's build is broken, or expand on a build. Especially if it's something like a main pathway.
Short answer: Yes, it's illegal. you may not alter a players build in any way, or you risk a ban. Long answer: Only the builder can decide what's 'broken' and what's not. You don't know what the builder wants, or the style/look they are after. And you also don't know what the builder is planning. maybe it looks griefed because the builder is 'renovating'. Or maybe the builder gave permission to someone to harvest items from the build for whatever reason. Maybe it's in the middle of being demolished for some purpose. etc etc. Bottom line is: Do not alter someones build (that includes placing or breaking a block). Or you risk a ban. But yea, most builders/owners of pathways don't mind people building off of them. But it's still technically their build, and altering can get you banned. So might want to ask them first? or leave a sign explaining or something? you know, curtsey. (I personally always contact the builder first for permission, and then build).
If you are having difficulty figuring out who built something, you could request that staff identify who placed the blocks. You might not get an instant response and older builds may be harder to identify. When in doubt, it's always safer to just look for a different spot.
This is getting really off-topic, but there's a pathway leading off the spawn of nearly every outpost entrance in either the wilderness and wasteland in nearly every direction, and it regularly gets demolished by creepers or griefed, and how on Earth can it be a fair expectation to figure out who the original creator of that pathway was in order to ask them if I can repair the hole or gap in the path that in the case of a skyway needs to be fixed in order to keep moving? If that's the expectation then I know people aren't doing it because I see pathways made of many different materials, and absolutely no signs next to them explaining that they were repaired.
And this is the dilemma. Things do need repairing, but you run the risk. And an issue raised often. For starters, you report the griefing of the skyway. Then repair it with same block, like it never happend? and chances are, the builder won't care. But you need to be very careful about what you do. Consider the possibilities, don't overdo it, try to use consistent blocks, don't make major alterations. However, I'm also regularly annoyed by people completely altering a pathway I built and closing off exits I had made, leaving paths open to hostile problems etc. I simply undo the 'damage', and leave a sign. If reported, staff have simply 'spoken' to the individual and not met out any serious punishment. It's griefing. But usually not with 'malicious intent'. *shrug* bottom line. you run the risk repairing pathways. But chances are, staff are intelligent people, and will figure it out. You shouldn't get banned.
