If Breyer had not resigned (and say died under the next President) would the confirmation still be valid?
Yes it is a first, and unconstitutional in my opinion, since the Senate confirmed a nominee for a position that wasn't vacant at the time of confirmation.
And if it is constitutional, then when the Republicans get the Senate and the White House, they should line up replacements for EVERY SINGLE JUSTICE and confirm them immediately. Hell, line up 20 candidates for the next 20 SCOTUS vacancies.
Where does the Constitution prohibit that? It just says the president will nominate and, with the advice and consent of the Senate, appoint Supreme Court justices. It says nothing on the timing.
It is by no means a first. It has become common in recent decades, because people are living longer.
Breyer was himself confirmed while Blackmun was still sitting (Blackmun had announced his retirement ‘pending confirmation of a successor’). Same thing happened with Sandra Day O’Connor, and a couple others I can’t recall now.