Just to demonstrate how non-sensitive DIY CO2 is to specific techniques:
I wouldn't sweat the 120 F thing too much- it's more to help the sugar dissolve than anything to do with aseptic technique. Among other things, the vessel isn't sterile, the inoculum isn't pure, and 120 F isn't going to do much for getting rid of bacteria or mold in any case.
In order to minimize the chance of mold contaminants, just work quickly and minimize the time you have the bottle opening pointed upward toward the open air and the amount of time the sugar/buffer mixture sits out. Mold is probably the one thing you should worry about, especially in humid environments.
I have never seen mold in any of my DIY CO2 bottles, and I also never make any effort to avoid it.
While alcohol is definitely an issue toward the end of the fermentation run, weak organic acid build up and changes in osmolarity are just as much of a problem for the yeast. As others have mentioned in their recipes, try a little baking soda and a pinch of salt.
Laying the bottle on its side will help with gas exchange, as will not overfilling it. Try to shoot for no more than 1/2 full, 1/3 full if you keep the bottle vertical instead of tipped horizontally.
I always fill the bottle up to where it starts to neck down or a bit higher. I have never experienced foaming of the solution, nor any problems from having the high water level. The reason I do this is to minimize the amount of air in the bottle so I get more nearly pure CO2 sooner.
If we were to keep the bottles upside down, of course with the CO2 tube long enough to reach the bottom of the bottle, that would eliminate any CO2 leakage at the bottle cap connections. The tiny amount of leak of the solution shouldn't be a problem. (just think about this for awhile!)
Don't even bother washing out the old bottle or changing the culture as long as its healthy. Just dump out most of the old liquid and pour in the new stuff. If you leave some residue in the bottom of the bottle, there will be plenty of yeast left.
I dump the contents, do a small flush with about 1/8 full bottle of water, shaking it good to remove the residue. New yeast is cheap, at 1/2 tsp per bottle filling.
As for the bottles, I shrink the gas separator
bottle down to thicken the plastic. If your dishwasher has a sanitize option, that should be enough to shrink it. I'm currently using a shrunken 12 oz Dr. Pepper bottle for that and a Schweppes Ginger Ale bottle for the fermentation vessel. The walls on the latter are thicker.
Carbonated beverage bottles are made to withstand pretty high pressures, so there is little chance that they will rupture from our use of them for our form of carbonated beverage.
For connectors, I'm using Foremost bulkhead fittings sealed with GE silicone caulk (the kind without the mildew inhibitors, as the chemicals GE uses work on yeast too). The bottles themselves sit in a plastic tub lined with old hand towels to absorb the gunk if and when there's some sort of catastrophic failure. I used to work with organisms that grew optimally at 30 atmospheres of pressure and 105C, so I'm probably overly paranoid in that regard.
My DIY system finally developed enough tiny leaks so it quit working. Each connection of hose to bottle has always had a very tiny leak, so I suspect those finally got big enough to shut down production. If I decide to do this again I will also try bulkhead fittings to eliminate that source of leakage.