It only takes a small leak over a period of time to drop the pressure so check out all the rad valves first.Also check out the blow off pipe(prv) that terminates outside.It could be the expansion vessel but check for leaks first.
Just wrap the short bits of unsleeved pipe in tape to protect from lime in the cement and fill.
Denso tape is what is normally used but any protective tape will do.
Use a pipe sealant if you get no joy with ptfe or remove olives,clean again and fit new olives with a smear of sealant on the olives.Copper olives tighten easier than brass
How much difference are we talking ?
I would cut the copper and put new pieces in but if you are not confident then it might be easier to drop the rad 1/2" or whatever.
Most rad valves will be the same length.