is there anywhere in the forums a brief walk through of the multi missions?
No, but I'm actually working to eventually add something like that to the wiki. It's slow going though, since I don't have a lot of free time at the moment and what I've done so far is by no means ready for public viewing.
It won't be real walkthroughs though, rather it'll be a list of the validated missions showing an overview like max players, max respawns, which race's ships you're flying, and an estimate of the mission's difficulty. From that list you can then click through to individual articles on each mission (I won't do them all, but I'll get the key ones done before I add this stuff in - Probably most of the ones that also feature on the medal list at the very least) where you can find details like loadout options, an overview of what happens in the mission, and a veteran comments section that can be used for hints and advice on how to beat it.
But again, that's a ways off.
and is there any way i can play the multi missions alone just to get a feal of them and without having to be online
You can play them alone, but you do need to be online unless you feel like editing the mission files. Just host a game as normal and commit without waiting for other players, and you can play alone. The stats won't save, but you can practise all you like.