Very very nice work! Looks like you ‘block’ your sweaters, the stitches are so nice and even and perfect!
Well, I like to use circulars even when a pattern calls for straights; the weight of the item is then on the wire and not my hands and wrists.
But to knit a “tube”, you want to make sure your beginning row is not twisted before you connect the first cast on stitch and the last cast on stitch. And you’re correct that PM = place marker.
I’d advise you to get a good quality circular; one with a very smooth join between the needle ends and the wire/cord.
The body of the sweater is knitted back and forth; when you come to the end of your row, flip the whole thing around and start on the other end.
When it comes time for the sleeves, you pick up the stitches you had put on a holder (I used scrap yarn in a different color and tied in a knot) and begin to knit in the “round” forming sleeve (tube). I didn’t have a short circular (and haven’t learned Magic Loop) so I used double pointed needles for my sleeves. It wasn’t fun. :teehee: But it worked. I also used the suggestion of rolling the completed sweater body up and tying up like a meat loin. That way while I did my sleeves, I could just flip the “package” in my lap as need be. And this also helped to make it a portable knit project (it traveled quite a bit between work and home).
I hope this is helpful; there are tons of others here who might be able to chime in on this.
Great job! 
My FLS has been hibernating for several months now. Maybe now I’ll get inspired to finish it!