"Nitpick: I see that sort of phraseology frequently from Orthodox on this forum. If the term "valid" isn't Orthodox, what term is?"
AB, my post 79 started with a quote from CC's post 76. His words not mine. That said, "valid" probably isn't a term an Orthodox person would use. The issue is whether or not a person has received, in one of the many "ecclesial assemblies", to use the Pope's terminology, one of the Holy Mysteries (Sacraments) of The Church. Assuming for the moment that such Mysteries can be imparted by any ecclesial assembly, and I think The Church believes that in some instances they can be, then the Mystery of the ecclesial assembly is effective to transmit God's uncreated grace. The reason that the word "valid" probably wouldn't be used is because some may feel it bespeaks a certain legalism which is foreign to Orthodoxy. In the end, and on this particular issue, I doubt the terminology means much as far as it goes, though it may imply fundamental differences about how we perceive what the sacrament in question "does" to us.
IMO, we say "valid", or more particularly "invalid", because "ceremony that remotely sort of resembles Baptism, is called 'baptism' by those who do it, but really isn't Baptism" is just too much of a mouthful to say on a regular basis.
Thank you. I am not fond of the word "valid" for the reasons you noted. But to be honest in English it probably is the closest term that fits.