I did my first felted joins recently, as well, and I'm really a fan.
As for the sleeve, did you try counting rows? Count the rows on the finished sleeve, and place a safety pin or something every ten rows. Then count the rows on the unfinished sleeve to see if you have the same number. That's the trick I was told to making both sleeves the same when you do them one at a time. Of course, I haven't made a sweater yet, but I just made a pair of fingerless mittens without a pattern, and that's what I used to make sure they were the same.