It is a very good idea to stop treating your lawn. That might be the key. Keep things as natural as possible, and see what happens.
What has affected his mouth and face is something he is routinely putting his snout into, and sniffing/snuffling in -most likely. The lawn may be the culprit.
It's wise to keep all chemicals away from our lives when there are dogs about. That can also include chemicals in home cleaning preparations (often dreadfully toxic!) which we think nothing of -but dogs are at a lower level than we are, and some of these things are heavy on the air, and hang around at floor level. Air fresheners/carpet fresheners are another thing to consider.
On most household cleaning products you will see hazard warnings/toxic warnings. Cut down the use of them, or limit them to places like the toilet etc, where dogs don't go near.
Steam cleaning everything is always a hygienic healthier option.
Skin conditions can be very difficult to figure out. Are you feeding out if plastic bowls? If so, switch to stainless steel or ceramic.
Thus could be food, a contact dermatitis, fungus or an autoimmune disorder. I doubt the anal gland issue woukd be connected. Your Vet might have to take a scraping ir biopsy to get a diagnosis.