1 and 2. By combining 1 and 2 so that you are rewarded for a friend who reached a certain rank (rewarding for every rank is too much) and limiting the number of rewards you can get. This way multi-accounting becomes too much of a hassle.
3. Like I said, it might not be viable so no need to pursue it.
4. I would leave it up to the devs to decide how to implement it but it's not a bad idea.
Regarding those who do not have friends, right now I have no idea how BFR is racking new players but I would assume some, if not most, heard of it from a friends or an existing member and since BFR is also a social game I don't see why this suggestion is bad. As for tutorial maps, I agree, it's a pretty good idea but you won't be able to add official maps so they would have to be custom tutorial maps.