I tore it all out and have now been reading up on the mattress stitch. I haven't gotten the guts to try it yet though because no matter how many pictures I look at or books I read, it doesn't seem to make sense to me.
Mattress stitch is fairly straightforward, once you get going. Sometimes it is easier to start about an inch above the bottom edge, leaving a tail that you can use to sew up the gap later. This eliminates the overtightening that sometimes happens at the bottom of the seams. An anal but really useful technique is to count up all of your rows on the front and back (count 10, leave a pin, count 10...). If one piece is a little longer, pass your needle behind two "bars" on that piece every once in a while, until you've evened out the difference. This will make sense once you've started.
There are great written instructions here:http://www.tradewindknits.com/tbmattst.html
With lots more good tips on the "refinements" page.
And there's a good picture here:http://www.wonderful-things.com/newknit10.htm
how the heck do you put in the sleeves? And does anyone have an opinion -- should I sew the sleeves in while they are open, then close up the sides and sleeves? Or close up the sleeves and side seams and (try) to put the sleeves in like a sewn garment?
Sew the side and sleeve seams closed separately using mattress stitch - and then swear never to knit another sleeve flat!
Setting the sleeve cap into the armhole involves some planning. It really helps to understand the "anatomy" of stitches.
Now I'm going to try to explain how to sew beautiful armhole seams. The seams you know your sweater deserves. Lots of people backstitch their armholes from the wrong side, and that works okay. If you want a really crisp, tailored, bulk-free seam, though, try it this way.
1. Like in sewing, identify the middle stitch at the top of the sleeve cap and mark it.
2. Tack the sleeve and body seams at the underarm. Look at how many stitches there are at the sleeve cap edge compared to how many rows there are at the armhole edge. The number will not be the same. There will almost certainly be more rows in the armhole. If so, then every once in a while you'll have to pass behind two bars (see above) at the armhole edge to even things out.
3. Start seaming. Remember: you're going to sew from the right side of the work. Start at shoulder seam and work down toward the underarm. Leave a long tail of yarn that you can use to seam down in the other direction. On the armhole side, run your needle behind the "bars" between the first and second stitch of each row. On the sleeve cap side, weave in and out of the stitches, as though you were grafting or doing duplicate stitch. Adjust the stitch tension as you go. Your work should blend in with the knitting.
4. When you get to the bound off stitches on both the sleeve and body at the underarm, graft them, Kitchener-style. The join will be invisible. It will look like one continous fabric. Stop sewing at the side seam. Gently stretch the seam you just completed and inspect your work. If you're happy with it, anchor the end of the yarn in the side seam selvedge on the inside.
I know this sounds like a big deal, but it is really isn't. Just go slowly and steadily and stay relaxed. And don't be afraid of undoing your seam if you're unhappy with it.