Need insight

They say to stir our primary country wines to encourage fermentation and reduce oxidation. I assume to not stir just before racking into secondary so the organic matter doesn't rot in secondary, is this right?
Then, if we have that lava "lamp action" in secondary, would that be rotting nasties into our cups?
I've noticed the volume drops during secondary where it first was gurgling into the air lock from 1" to being over 2" below the airlock..?
Should we make our home grown recipes to fill the carboy and then dreg into a few litres extra of spare bottles to make up the space taken by biological processes; being gas, heat and reproduction? Thus giving us exact same wine to top the batch up with?

I can only assume the drop in volume in secondary is from the using up of sugars, seeing as the adding of table sugar is adding into the recipe volume as a whole.

Not sure who they are, but stirring during fermentation does a bunch of stuff. When fermentation is over, I rack my apple wine without stirring to get rid of the course stuff. Volume loss is basically the volume of the gas that is going from the wine and out the airlock. Once it is still, no bubbles per second, the volume should stay pretty constant in glass or plastic.

I'm guessing the "lava lamp" is the continuation of fermentation. If it smells good you are ok. Definitely top up that air space as it goes still. If you have a good tight airlock, it should be mostly CO2, but oxygen sneaks in and that is what feeds most of the " nasties". You can use more of the same, I try, or a similar wine, glass beads or as a very last resort, distilled water (ugh). You can also rack it into a bunch of gallon jugs and various bottles. Good luck!

Great idea using marbles for displacement! Thank you!