You can find a good indication of how much something shrinks when felted in this thread
Remember too that you typically will use a significantly larger needle size for the yarn you are using than you normally would if it was a non-felted item. Any felting I've done with worsted weight yarn was done with at least a size US10.5 needle. The loose knit fabric is important for the felting to happen properly.
As for the eyelets, you could do a double YO, just remember to compensate for that with the appropriate decreases, and don't worry too much about them even with felting - you can actually use a scissors to cut fully felted items and it will not cause it to unravel. So if your eyelets felt shut a bit, just snip them back open.
I'd knit and felt the strap seperately and thread it through afterwards.