I'm a little worried/disappointed because I have a feeling I know the answer, but here's the situation.

I completed the quest during which you escort Serana back to her family's home/castle. Of course as a result, I was approached by Lord Harkon and offered the gift of Vampirism. Prior to even activating this quest, I completed the quest (I don't remember what it's called) between the Werewolf Sinding and Hircine. After killing the Stag in the quest, I went to where Sinding was hiding, I was given the option of killing or sparing Sinding, but could not decide which to do. I looked up the perks of both options, but forgot to pause the game, and the conversation timed out, and Sinding retreated back behind the cliff. I then made my way around to find him and re-obtain the dialogue. As I found him, he ran past me, and began killing hunters. I stood by, and attacked neither Sinding, nor the hunters, but after he killed all of the hunters, he turned to me with "You shouldn't have come here." and attacked me, leaving me no choice but to kill him. I attempted a similar strategy in the conversation with Lord Harkon, first on accident again. I was looking up the perks of Vampirism, and the conversation timed out, and I was kicked out of the castle. However, I was for some reason still able to reenter the castle, with Serana behind me as if we had just arrived, though she wasn't when I was outside. So, deciding that I didn't much like the idea of not being able to experience daylight without ruining your life, I timed out of the conversation again. (this time on purpose) due to the different circumstances. I assumed that once I decided whether or not to become a Vampire, I could return and reenter the castle as before, since I didn't actually say "No". But I couldn't. The man is not behind the gate to talk to, and I ran around the outside of the castle looking for another entrance, and found one with Skellies and Draugrs outside, but it required a key. I ultimately went to Redwater Den and became a Vampire, hoping it was a roundabout way of gaining reentrance to the castle, but it didn't work.

After this very detailed explanation of my situation, basically what I want to know is, since I didn't actually say "No" to Harkon's offer, is there any way I could gain entrance to the castle to obtain the dialogue and accept his offer? I decided afterwards to become a Vampire until I obtained Auriel's Bow, then, if my mind didn't change, to be cured. (I know I know the effects of the sunsoaked (or something) and special elven arrows) But my main weapon is the bow, specifically the Zephyr from the Aetherial Forge quest, and I wanted the artifact. So can I reenter the castle and complete the conversation, becoming a Vampire and blah blah blah eventually getting the quest during which Auriel's Bow is obtained. Or, if not, is there any possible way I could still obtain Auriel's Bow regardless?

You get Auriel's bow from either side of the quest. join the dawnguard.

If you've been kicked out of the castle and gotten a quest update to report back to Isran, you'll have to join the Dawnguard to complete the quests. You're going to have to cure your vampirism first though, or they won't let you help them.

The only way you can join Harkon's side now is to reload an earlier save and replay that part of the game.

So I can get Auriel's Bow from joining the Dawnguard as well? That's reassuring. And I suppose I'm going to have to go talk to that creepy Oblivion-vacationing elf and get cured. And there is now no way I'll ever be on Harkon's side because that was a while ago, I just thought of asking a forum now, and have moved on from that dilemma. There's no turning back. that was like 5 levels ago. But I did a lot in those five levels. Also in the quest for S(something) Signus guy north of Winterhold, I somehow think I accidentally left the Lexicon in the pedestal thing at the location of the Elder Scroll, because the "Transcribe" or "Inscribe" the Lexicon part of the quest isn't complete. Does that mean I'm going to have to go all the way through Blackreach and find it?

And because of the Sinding incident I described above, how could I become a Werewolf now?

Sinding's quest wouldn't have given you the opportunity to become a werewolf anyway, only way to become one is to join the Companions in Whiterun.

Join the Companions. You'll have to become a werewolf if you do enough of their quests. That will also cure you of your vampirism without having to visit Falion in Morthal.

Joining the Dawnguard will also allow you to get the enhanced crossbows and crossbow bolts, which you can't get if you join the vampires.

If the lexicon is not in your inventory, then you're going to have to go back to Blackreach and get it. If you opened up one of the great gates on the surface when you left, you can cut down considerably on the amount of backtracking you'll have to do.

I don't know I might just join the Dawnguard and kill myself XD. What would have happened if I was able to spare Sinding? And I know I wouldn't have become a werewolf, but he had the ring that let/helped you control the transformations, right? That was the main potential benefit I found in Lycanthropy, so not getting it kind of killed it for me. And what about the Lexicon situation?

Sparing or killing Sindig only determines which Daedric artifact you get from the quest. If you spare him you get the Ring of Hircine, which is actually cursed when you first get it and causes werewolves to randomly transform into Beast Form (once Hircine lets you keep the ring at the end of the quest, the curse is removed and the ring can be used to allow more transformations into Beast Form than the one per day that the original Beast Form power allows).

Killing Sindig grants you the Savior's Hide, a Light Armor that gives you 15% magic and poison resistance. It's possible to obtain both rewards through an exploit, which you often have to do anyway if you're trying to get the Oblivion Walker achievement, thanks to a bug introduced by Dawnguard that causes one of the other Daedric artifacts to not register when you acquire it.

Yes I was planning on doing obtaining both but he attacked me and at that point there was nothing I could have done about it. I tried leaving without attacking him and everything but it didn't work

I know a decent bit about the game but for some reason in Elder Scrolls games I always manage to find roundabout ways to mess things up.

Also. Another important problem I have that I can't in any way explain, involves the quest to defeat Rahgot. I stopped playing Skyrim for a while and in the past few weeks began again. When I started playing again, I was level 17, in the structure where you find Rahgot. However, after fighting through the entire structure, searching every urn, chest, shelf, etc. I could see, (mostly out of habit) I reached the long tunnel to the dragon claw door. I believe it is for the Glass Dragon Claw. The problem with this, is that I don't have it, and it being a quest item, which I can't get out of your inventory, I never did. So, I turned around to go back the way I came and look for it, around just two corners there's a spear gate that appeared and blocked my way out. I looked all over next to it and in that part of the tunnel for a pressure plate, lever, pull chain, button, and there was nothing. I was literally physically trapped and had to reload a previous save. Any ideas what happened with the claw? and why I got trapped?