Septic system debates are great because almost all of the information is anecdotal, as is mine.
The way I look at it is when they backfill the septic systems they do it with a dozer that usually weighs much more than a B or BX, and applies more ground pressure even if you consider the load distrubution of a tracked machine.
It is all about soils conditions, both for structural support and the effectiveness of your leach field, but I think that generally any of these mowers can go over almost any septic system. If you have concrete galleries I wouldn't hesitate, perf pipe in a trench should be fine to. Generally, these materials are strong by virtue of the backfill around them and the subgrade, a failure of that fill from a washout or wetness or of the material below could lead to a broken pipe, for example, if it is really wet and you sink in above a pipe you may have trouble.
So I think, as with most things, a little bit of judgement as to the soil conditions at the time and you'll be fine.
Regarding using bacteria stuff in the tank, I admit that I do and can't say that it works. It is the action of the bacteria in the tank and at the edge of the leach field that cleans up the water. I wouldn't do it but you should be able to drink the water just a few feet from your system once it is filtered through the bacteria and earth.
I sold a house a short while ago that had a septic tank in the ground for 11 years and we never did anything to it, the guy came out to pump for a home inspection and said that we were wasting our money pumping it out, it looked like new.