Whether NATO is or isn't meaningless, is its purpose to defend all of Europe?
It's purpose is to defend its members.
Is its purpose to continually grow and expand membership?
Its expansion or contraction should be carried out on a case by case basis. Generally arrangements like NATO work when the nations involved are generally democratic and when the people in those nations have mostly common values. For example, though I think Romania and Poland are good fits for NATO, I don't think Turkey is. In fact in a way, Russia would be a better fit in NATO than Turkey.
If not, how wise is it to follow that trajectory when Russia announces they plan to oppose those objectives militarily?
It's a factor, but you don't let that dictate the decision. If you do, then you aren't sovereign. You're a de facto puppet of Russia.
And if expansion of NATO is an objective in terms of protecting Eastern European countries, how much safer are these countries now that Russia has labeled them a target as long as the ABM system resides there?
That depends. If you accept the premise that Putin is the epitome of morality and virtue and that his concern is truly nothing more than self-defense and that no other nation will ever be missile threat, then it's obviously safer not to have an ABM system. It takes a lot of naivete to buy all that, and most European nations aren't that naïve.