How to seal PVC-U and copper pipes together
I too have struggled with gluing copper and PVC for certain experiments. Silicone is an ok choice, depending on the pressure, but white silicone just happens to be less effective for sticking to stuff than clear silicone... I don't really know why, I just know that it is.

It's kind of too late to use anything else now that you put silicone on it (actually PVC cement seems to stick to silicone, but it's a poor glue overall); but chances are the white silicone will separate or peel off after a while. If (or, after) that happens, you will have another chance to use another glue. The main thing to be concerned with is gluing to copper. I think that Just for Copper epoxy would be a better choice. I have not used this before, but based on round-about experience, I think it would be good to try. In general, copper oxidation (patina) is a problem for glue on copper (in the long run). Gluing to PVC is probably not going to be as much of a problem... at least for most kinds of glue, silicone, and/or epoxy. Maybe the the answer is to use something like a copper putty to adhere to the copper, then use silcone or another epoxy to join the PVC to the copper epoxy.

Something else that might help would be to use a piece/sheet of plastic or rubber over the area (like a shield). Use something, in which you can cut tight, accurate holes to help seal the deal. Some plastics don't stick to some epoxies. So if you decide to try this, test the plastic for compatability first.

Edit- See where I think a weak spot could be; unless maybe another pipe is inserted which helps support this area.