If I RS a player and he drops to 1 WE, will his skills drop massively or will they stay about the same point where they were at the start of the season?

In my opinion, it will ruin the player.  During the RS season his skills may improve a little with practice, but during the offseason they will drop more than they have improved and he will never recover.

The skills won't drop massively, but he won't go up much, if at all, and will lose whatever points he gained on rollover.

He's a transfer and already has all the stats that I want him to have, I don't really need him to improve anymore, will he for sure transfer if I RS him and he goes to 1 WE?

No, I don't think you can necessarily assume that he will transfer, but I understand that there is a risk.

He's unlikely to transfer, but you do need to be aware he'll lose a few points here and there in the offseason.  If he's what you want now you can probably afford to RS him.  Of course, since it can't get any worse than dropping to 1 this would be a good time to leave him off the depth chart and try taking the RS off and putting it back on and see if you can get even a slightly better result.  If you get the guy up to 8 or 9 WE you'll be more likely to break even between the RS season and offseason.

He's unlikely to transfer, but you do need to be aware he'll lose a few points here and there in the offseason.  If he's what you want now you can probably afford to RS him.  Of course, since it can't get any worse than dropping to 1 this would be a good time to leave him off the depth chart and try taking the RS off and putting it back on and see if you can get even a slightly better result.  If you get the guy up to 8 or 9 WE you'll be more likely to break even between the RS season and offseason.