i can help with some (but not all) of those, not sure if they're right, but they are my understanding of them
Inset - inserts the object a certain amount of pixels, so if you assign left and inset 10, it'll be at the left-hand-side but 10 pixels towards the right.
offset - if you arrange vertical two containers could (for example) be laid out taking 50% of that container space each. offsetting will an object to be moved off the centre of it's object.
gap the amount of space between two objects that are layed out. you can have -ve values too, so that it'll bring two objects closer together (look at picking.xml).
arrange_horizontal_attachment (and vertical). these you can use to say if i've got 5 objects, arrange them across so they are all on the same row, and can allocate a certain amount of space. best use i've seen of these is to sepecify layout="-1,-1,-1,-1,-1" and it'll layout 5 objects evenly spaced across regardless of the screen width. Sometimes you want something stuck exactly where you want it so you may have something like "-1,-1,30,200,45" which means the first two objects will take up any remaining space, and the last three objects will specifically use the 30,200,45 spaces.
This can be used to layout just about anything, along with vertical and placed inside each other
they are the crux of what i use to get the layout i want.
stick_to_sides use this to have an object in a container align itself left, right, top or bottom. or a combination of both. if you want it to the left, inset 20, but don't want it inset 20 from the top, specify two seperate lines, one for the top with inset="0" and one for the left with inset="20". or if you do want left and 20 insetted 20, then you can combine those lines to say align="top,left"
for guides and xml - there's been some posts similar to this, and was a stickie for a while that had people asking skinning questions and responses. For xml guides, there's generic doc, just use google. I think a good editor can help you learn, as you can see clearly what is subsections etc, but I still stick with notepad for all my stuff.
Start by copying and pasting small sections around, trying to relocate stuff. That way you can learn how things are grouped etc, and start to visualise the code, which helps.