It is probably tight as can be. I had a similar issue on an older one and it took a lot of grunting to get it to move. This picture is a but confusing, is this sideways and is that the bottom of a sink or of a water tank?
You could always just cut the two pieces of PVC where it says “E84” then pull the whole assembly so you can work on it. Then simply put it back together with two shark fittings. No glue or anything necessary.
There is also the option to just replace all of that PVC with PEX while you are doing this. It’s a really cheap and easy material to work with and you do not have to worry about winterizing it. It also bends pretty easily around corners. The shark fittings if you use them just snap together and can be taken apart as well. Or you buy the cheap ones you just crimp on. Just a thought.
Yes pex can be connected to pvc and copper. You just get the shark fitting for that application. Plan it out first, the fittings are about $5 a piece if you get the ones that snap together. You really only need a couple though.
It is a little hard to see here. But this is a piece of pex with a crimp on the bottom and a shark fitting in top.