I flew JFK-SIN and back earlier this month. I was able to grab one of the solo seats at t-48 since one became vacated. It was my second trip on the A350-900ULR.
I loved the bin but I felt my feet were too restrained, maybe because I accidentally put up the leg rest. In a noob move didn't realize I could close it. More likely the equipment box. Last time when the product first came out, I flew I switched to a set of 4 w/ empty middles after boarding which I preferred better.
I didn't notice any noise from the lav, which is behind the rear exit on the C side (I was 41C). The third lavatory behind H seat I'm not sure but definitely avoid last row of the middle section.

The return flight was nearly full so I bought 31A for $130 (completely full on departure). I think I prefer this seat the most. You have the window to lean against and you can get up without disturbing your neighbor. The biggest downside of not having the solo seat for is if you order a special meal or BTC, which are served first, you are trapped in your seat waiting to get table cleared. If you are in solo just put the tray on the bin cover and go to the lav. I'm 6' and was able to stretch my legs out without touching the exit door. Since I have long arms, I loved the grab bar next to the exit which helped me in an out of my seat. The foot rest styles for both are useless to me.
There wasn't much congregation or stretching in the aisle. The FA did chase down a few pax trying to use the J lav so it was pretty quiet after that. The curtain and the plane sounds were enough that I never heard the lav door or flushing. I maybe smelled a whiff of something for maybe 1 min of the 19 hour flight.