I have never experienced a plastic valve on a copper line. Plastic fittings are made to seal on plastic material. The reason for this is to eliminate the necessity for using other fittings such as Reneros. Without the Reneros, the plastic will not seal. In the early days, we used to use the Reneros in these applications. This is the proper type of fitting to use on copper with a brass casing. These will make working with a brass casing much easier. Some prefer to use brass fittings on 1/2" copper and 1/2" CPVC. I would recommend to avoid this type of connection.
The T-joint supplied should work as well as the other round stuff, the core of the coupling should expand into a matching female hole via the plastic female connector. If the core of the coupling is too large, the plastic female connector will most likely be disengaged and all pressure will be lost.
I just unplugged my existing diverter on my deltap 500 bath and installed a bypass diverter from the new valve. It is a larger diverter but works well. I had installed this valve about a year ago on my deltap 400 bath and it worked fine. Now the new one has stopped working. It is a very large diverter and this is the first time I have had to use a bypass diverter. I could use a different size but it has a lever that I cannot find anywhere that is back to it's original position after installation. Worrying about the wrong size diverter could have saved me a lot of time and money. Should I use a different size diverter or is there some way to reset this diverter? d2c66b5586