The Paladin nerfed two near-lethal unarmed strikes with necrotic damage by applying the "protection" fighting style (giving Strahd disadvantage)
It sounds like the Paladin may have been doing this wrong. You only get to use it once per round, and it costs your reaction, and you have to do it before Strahd rolls his attack. You can't wait to see if Strahd hits or misses--you have to choose to do it right away, and since Strahd is making three or four unarmed strikes per round, and only one of them will be at disadvantage, it should have minimal impact.
The Strahd Zombies that arrived with Strahd were entertaining but fodder.
Best used as a distraction to pull the PCs out of position while Strahd (under Greater Invisibility) slips behind the back lines to mug the wizards and clerics.
Round two he summoned five wolves. Seems way under powered for the supposed 9th level encounter, one Fireball blew up every wolf, I had them arriving in a pack intending to use their pack sense for advantage.
Waste of an action. He should have summoned them before combat, or after leaving combat but before returning with a second wave of wolves. Summoning wolves in combat is way too late. BTW, 5 wolves is a really low roll for 3d6, so partly the PCs just got lucky. (They also got lucky that the wolves arrived in Fireball formation.)
I failed to break the link with the heart of sorrow, something I had to do on Strahd's turn as a bonus action, it was here I think I knew I was in trouble.
I used the legendary actions mostly to first move out of an area affect, and second and third actions normally the unarmed attack, which I assume is one attack.
If Strahd thinks he is at all in trouble, you can use Legendary Actions to move away from the party, then Hide on Strahd's turn (with his +Enormous stealth modifier) and regenerate/summon reinforcements, then come back under Greater Invisibility.
For spellcasting, I assumed one casting of a spell took up one of the two unarmed attacks, as it would a normal "action". Blight was near lethal.
Spellcasting takes up his whole action, i.e. he gets no unarmed attacks that turn. But he does still get Legendary Actions.
Blight is a complete waste of a turn. He probably does more damage than that with his unarmed attacks.
I regenerated Strahd 20 points after every round, but did not regenerate the heart of sorrow. Is that right?
Yes, that's right. But Regeneration is at its best when you're using it to hit-and-run.
Still, you can explain Strahd's suboptimal tactics by just saying, "He was arrogant and got careless." So the PCs killed a vampire and good triumphed happily ever after. The End*. Nothing wrong with that.
* Except that in Ravenloft, happily ever after is never The End. But the players don't necessarily need to ever find out about that.