But once you get to the Castle its strictly Metroidvania all the way.
But the castle in OoE is actually still rather linear beyond being able to go to either
Eligor or Death after Blackmore,
so I'm not sure I'd consider it strictly MV. There are some side passages and round about ways of course, but they're rather short and either quickly turn into a dead end or return to the main path rather than opening up into a more open ended affair (like what some of the potrait worlds were like in PoR). Part of it may just be because the castle isn't as large as incarnations from between SotN and DoS, but I still got a pretty big vibe of stage-based progression even in the castle, even though you back tracked a few times.
I thought what the game wanted you to think happened to Richter was infinitely more interesting before Shaft shafted his way back into the plot.
This is basically exactly what I thought of SotN's plot.
Richter trying to make his battles against Dracula last for eternity is naturally a much more interesting plot than just Shaft coming back and corrupting him.
Still, I can see why they felt that wouldn't be an acceptable path to go for the "true" plot. I just like that they injected it and gave us a taste of something like that despite going a more traditional route in the end.
Anyway, how Shaft got back into action leaves a lot of room for fan speculation but I don't necessarily think it was required for them to explain it--some games do leave stuff like this unexplained to add stuff for fans to decide on or make up themselves. Perhaps at the end of Rondo, in his fight with Shaft's ghost, Shaft had some kind of final trick up his sleeve where he subtly attached his ghost to Richter with the last of his energy and appeared to die, allowing him to gradually regain strength and corrupt Richter over time. That's about what I get out of it if I just look at what happened and need to think of a way for it to make sense.
I'd like to think just gathering Drac's remains together would summon Castlevania, but you're right. That's a guess/theory.
That is likely actually true given another one of the Metroidvania's plots, but it'd probably be spoilers to say any more. How you thought of that without playing that particular MV is actually rather perplexing to me.