As Jenn and Shadowcat both said, have your placket/stomacher stitched into the Right hand side seam and hook it up on the left hand side seam. (assuming you are Right Handed!). You can then pin it through on the left hand side (probably will need help with that every time you need to dress in it!) so it looks more authentic.
Take the costuming a tiny bit at a time otherwise you will get totally overwhelmed!! Don't worry about the stomacher/placket bit at this stage. You need to concentrate on getting the bodice properly fitted first.
As regards where the bodice sits, i will happily take your word for it that it needs to sit properly on the waist before the skirts are attached, but surely the weight of the skirts will pull it down a bit anyway? or am i missing something about the method of attaching the skirt?
Yes, the skirts will pull the bodice a certain amount, but when a bodice fits very closely and is tight enough to support you (I assume you know I mean here!), then even the weight of the skirts shouldn't shift the bodice in any way. Also, there is a method of attaching the skirts which logically should not work but for some reason does! Don't worry about that for now though.
Regarding tightening the straps once the sleeves are on, this is a bit difficult to explain withut pictures. I will need to use my dummy to get a photo so you can see it but you will need to wait a bit for that - my sewing room is resembling a costume explosion at the moment and I can't find anything at the moment!! Again, dont worry about it at this stage.
Anyway, its 10pm and I need to go to bed as Im awak at 5.45 tomorrow morning (I hate commuting!!).
Take it all a small step at a time.