My solara project is several years old now and has some fusel notes. What I did not consider when I was adding beer to the vessel (it's in a carboy) was that adding wort to it would keep increasing the alcohol of the main batch. I've learned now to add fermented beer instead.
How would this be the case? Wort or beer, its all going to depend on sugar and volume. Although there is some concentration during aging, my wine needed about a 500ml addition per month. I didn't top up this barrel, I fermented a batch in it then filled it with fermented beer a couple weeks later.
Hmm. Rethinking this tells me that you're right, the alcohol % is not increasing unless the unfermented wort is increasingly higher in gravity. Warm fermentations caused my fusel notes. Sorry to sound stupid.
Unfermented wort added to the solara will take on the characteristics of whatever yeast or bacteria is already in the solara.
Mine has a huge layer of trub at the bottom, but there seem to be no notes of autolysis.