Hello: Just off the top, it sounds like a plumbing issue more than a water heater issue. Pressure should remain nearly constant. That’s an adjustment in the pressure switch or pressure tank concern. By letting pressure fall, I’ll guess smelly water from a dead leg in the plumbing (or possibly off the bottom of a tank) is making it’s way into the system. This gets flushed out with use. So, if there are no dead legs in the plumbing, you might look at how to prevent back flow from the tanks.
Regarding your second question, the anode can be removed by tipping it back and forth to squeeze the anode together as it’s removed. Sometimes aluminum rods split lengthwise and the tipping, helps.
About question #3, it really depends on where the odor is coming from. I would run some water from the bottom of each tank and sniff it. If you pass out, you know you’ve found the problem 😀
About questions one and four, I think they both could have to do with the pressure switch/ dead leg issues I mentioned at the beginning of my response.