It seems you need all five body parts and the magic cross in order to destroy the blocks on the bridge leading to Dracula's castle. Even if you have ring and the nail, nothing happens if you try to destroy them. I'm not sure why specially Dracula's ring is the item that is described as granting entrance to the castle if you actually need all five body parts in order to proceed. Unless I missed something.
I should mention I made use of a password generator in order to start with all the neccessary items. I'm not sure if that affected anything.
Re: the bridge, you either need to equip the Nail and whip the blocks (referencing the clue saying something like "Dracula's Nail may solve this riddle") or use the holy water. (I've always used holy water, and never even knew the nail was a thing. Which actually doesn't make sense thinking back that if you use HW you don't need the Nail selected, it's only for whipping blocks!! Wtf???)
I suspected there'd be a fail safe that either wouldn't allow the player to collect the cross or access the bridge without all the body parts.
My 2 cents: I'd just like to say that from a narrative perspective, given Simon requires all the body parts and the cross to "see the path" to Dracula's Castle, it's so weird that there's a final body part that Simon doesn't, nay can't collect because it's in the damn throne room... While mediating between narrative and gameplay, it makes no fucking sense!!