I'm pretty frugal in reusing things. I would not reuse the water from this, even for cleaning. Just my opinion. You could cool it and dump in the garden.
Three buckets. Drill holes in one. Place that bucket in one of the others. Put bag into the bucket with holes. Take third bucket and squeeze down on bag.
What is the flow rate on your pump? I have the same setup and I think I'm doing 1-2 gallons per minute. That pulls wort across the element keeping it from heating too much. If you circulate without heating you'll find your temp drops a good amount as it is being pulled through those hoses and...
If you aim your return downwards, couldn't that lead to the liquor at the top being cooler than the rest? My return comes to the top and while it causes a whirlpool, it doesn't really disturb the bed below. Maybe I need to.
I wonder if the diameter of the pot in comparison to the amount of grain has an effect. In fact, I would guess it likely does. The larger the stack of grain is because of either the grain volume or the smaller diameter of the pot, the more likely it is compressed and possibly sticking.
60 is enough for conversion. More likely you had your mash temp too high generating too many unfermentables. I can't recall, but there is a product or two out there that can break these down and make them fermentable, but you run the risk of drying it out too much.
The D180 would not cause a higher finishing gravity as it is mostly fermentable. Did you make a starter with the 099 and pitch it at high krausen? Doing so gets the yeast to work at its most active time.