Questions regarding my plan for new 50A circuit (with pictures)
Answering your questions in turn:


You will be using individual wires in the conduit here -- there's no reason here that merits wrestling a fat, annoying cable down a conduit!  All four wires, however, will get pulled through the conduit as a group -- you can suck a string (tie a plastic bag on the end) through the conduit once it's together that you can then use to pull the wires through.
You are fine on the neutral here -- a meter for 120/240V split-phase service doesn't need it.  Simply nut the line and load neutrals together with a suitable wirenut (or a mechanical screw-terminal splice connector) at this point.
Your proposed grounding scheme is also fine given your choice of conduit -- I would suggest running a bare ground wire instead of an insulated one, though, as that saves a bit of space inside the conduit.
Since you are running conduit through the studs instead of a cable, you will not need bushings of any sort to run the conduit through holes in the studs.  I suggest running the exterior conduit above the window, by the way; that way, you don't have to try to fit a conduit behind your gas meter.
You do need to secure the conduit to whatever it is sitting against at 3' intervals, yes.  Strapping to every other stud works if you are running against the siding with studs 16" O/C -- for studs 24" O/C, simply attach a conduit strap to each stud, through the siding.  If you do decide to attach the conduit to the masonry, use masonry anchors or fasteners for the conduit straps, on 3' spacings.
1" is a suitable size for the run you have planned, but PVC may not be the correct material -- you may prefer EMT instead, although you'll want to transition to LFMC for the compound bend at the garage-end of the outdoor run instead of using conduit bodies for that -- either that, or bend EMT or PVC to fit using a conduit bender.  
If you're going to do the run over the top of the window, then I'd mount the meter pan so that its bottom edge lines up with the top edge of the window.  That means you can simply run the conduit out the knockout on the left side of the meter pan.