Are the connectors completely different? Can you post pictures? Or are you annoyed that the gas posts and liquid posts are different sizes? If the latter, check if there is a notch on the hex head of the gas post. That’s how I tell the difference between liquid and gas on mine. You could also...
You need to use more soap or starsan in the mix then. You can also make it sudsy and put the bubbles on there. If it’s a super slow leak then it will be hard to catch. You could also submerge it in water 🤷
The PSI will go down if you disconnect it as the CO2 gets absorbed but at some point it will equalize. I’d expect there to still be pressure even after 10 days but it won’t be fully carbed and it won’t be at the pressure you set because you didn’t keep it connected.
You can pressurize the keg...
It sounds like you might have a leak in your keg then. I’d take off the o-ring gasket and give it a good wash and inspect it for damage. Also wipe around the lip under the keg to make sure where the gasket seats is clean. Then, wet the gasket a bit (or use keg lube) and pressure test the keg again.
I have done BIAB and 3V but recently went 2V. Basically just ditched the HLT. I find that I get clearer wort being able to lauter. It works well for me. I still mash in a bag just so cleanup is easier though haha.
I don’t think you necessarily need to subway tile the whole thing. I think a backsplash over the brew stand and sink might add some good texture to the wall. I did that for my setup and it has made it really nice to clean. Mine is also in my garage so I know what you mean about wanting it nice...