.... I'm assuming the "breech face" is simply the wall of the slide where the firing pin pokes thru? If that is correct, would it be safe to use a toothbrush with a bit of solvent on that area to clean it?
Yes, that is correct! (Don't worry about pointing the slide toward the ground. It should be off the frame anyway; so, point it anywhere you like.)
(While pointing it to the ground, then of course drying it thoroughly?)
You don't have to worry about drying your slide thoroughly either. Excess lubrication simply isn't THAT critical. (You're being too cautious and overthinking this!)
Also am I supposed to "retract" the firing pin before cleaning that area? Or do I leave it in its "struck" position to keep the hole better sealed?
Yes, if the FP is protruding out of the breech face, then, give the FP safety a tap and allow the FP to retract back into the FP channel where it belongs.
If anyone can go into more detail for what is safe and what I should not do I'd greatly appreciate it. The manual that came with it doesn't go into very much detail at all and I'd like some info on how to safely clean that rear area of the slide.
You need to learn how to disassemble your new Glock's slide. I would recommend that you clean out the internal FP and EDP channels, at least, once every 1,500 rounds. I like to use either Q-Tips or soft pipe cleaners.
Other than this, just clean your Glock as you would any other pistol; and, then, lightly lube it in accord with the six lubrication points as they are shown and highlighted in your owner's manual. As long as you don't pour copious amounts of oil into the FP channel, you will be fine.
These things said: It is impossible - absolutely impossible - to keep all dirt, oil, and firing residue out of the slide channels. This is the reason, 'Why' there is a little weep hole on the bottom of the slide immediately behind the breech face.
If you need to learn more about your Glock, how to run it, and what makes it tick, I'd recommend that you add the following link to your Internet Explorer's Favorites list.
Illustrated (Unofficial) Glock Armorer's Manual
This manual is beautifully illustrated and takes awhile to load; but, the wait is well worth it! If you need to close it out quickly use, 'CTL+ALT+DEL' and shut it down with the Windows Task Manager.
By the way, if you don't care to use anymore light gun oil on your Glock than you have to, try using a high tech grease like Sentry Solutions, 'Hi-Slip' on the rails, connector top, and barrel areas. You'll need to wear Nitrile or Latex gloves if you do this; but, you should be wearing gloves whenever you clean a firearm, anyway. Hope this helps you out!
Sentry Solutions