Surprisingly, I don't know any Philips/Phils so I can't say for sure but I guess it's avoidable. Like you meet somebody in the market and he/she asks his name, you say: "He's Philip. Not Phil but Philip". And politely remind everybody who calls him Phil that her prefers(you prefer) to go by Philip.
And I personally like this name spelled with double "l" though I think that's just me.
I think it is avoidable, definitely. Families are usually the people that start nicknames, make sure your family knows not to start that one!
I went to school with a boy who only went by Phillip. I like both spellings, but would probably go by which is more popular.
I think Philip looks a bit cleaner and more masculine.
I know a handful of Philips who all go by Phil. When it comes down to it, as your child gets older it's going to matter what he prefers. There's a good chance he'll prefer to go by Phil if you choose that name, but if family and everyone else calls him Philip as a child, it might not happen.