1. Can i unlock the spartan officer achievment by playing lone wolves ?
Yes. That's the short answer. The real answer is that this achievement is now glitched, but can still be earned, and has been glitched since TU2 (the last patch). Now, you have to get to level 10 (skill) AND earn 50 exp to get the Lieutenant badge in a single playlist.
As far as I know, you can do these two tasks in separate playlists. You can get to rank 10 in Lone Wolves, then win 50 games in Big Team to earn the Lieutenant badge, and it will unlock. Or you could just do the whole lot in one playlist. Up to you.
2. When i play lone wolves i usually get 1EXP every game, does this get harder to get EXP the higher i rank up, or if i dont get a certain amount of points do i not get EXP.
You will always earn 1 exp for winning (unless you're playing in a weekend-only double exp playlist, where you'll get 2!). You will always lose 1 exp for quitting. When you quit (or disconnect), the negative exp won't show up until you complete your next game. Also, I believe you can quit Rumble Pit and Action Sack without being penalised.
Rank, which can fluctuate between 1 and 50 depending on performance, becomes harder to change as you play more and more games in a particular playlist.
This link explains why:
TrueSkill Ranking System - Microsoft Research
In other words:
Rank is variable.
Experience is cumulative.