Relax...this is called a quarter-life crisis. People who are too young for mid-life crisises (crises?) have them periodically. :)
You're freaking out because you're going through a period of rapid change in your life. You're getting married, graduating, looking to find your first job in a crappy economy, and your family is struggling - it's no wonder you're second-guessing everything. Take some time to relax, and once you've calmed down, ask yourself about this stuff one thing at a time. Don't try to tackle all of it at once, or you'll just get overwhelmed again.
1. Take a deep breath, close your wedding binder, hide all the magazines, and don't think about weddings for another month. Put all wedding stuff on hold. After a month, come back to the issues you've posted about and see how you feel with (hopefully) a clear head.
I'll talk about the money issue when I get to Part 3, but let's talk about the number of guests for now. Could you feasibly cut the guest list? Have you sent out STDs or otherwise informed all 150 that they were invited? If not, then you can still make changes. How does your FH feel about this? Personally, we had 80 guests at our wedding, and it was just what we wanted. I suggest that you go through your guest list and really think about each person on it. In 20 years, will you regret not inviting this person to your wedding? If not, then you probably shouldn't be inviting them.
About all the rest of your comments on this...you're really overthinking this. I'd forget the idea of a destination wedding, eloping, changing the date, etc. There's a reason you originally planned it this way. The only things that I'd change (if you want to) are the budget and number of guests, which both affect each other anyway. And don't worry about the weather. You obviously picked a winter wonderland theme because you love winter...so wouldn't snow be a good thing? Your closest family and friends will make it to your wedding even if they have to crawl through a blizzard, so don't worry about it. Anyway, my point is that you are overthinking this - majorly. If you continue thinking, 'Well, what if I did insert random different choice...' you'll drive yourself insane. Stick with your original plan when it comes to the date, decorations, etc. Scale back on the number of guests if you need to.
2. So move 2 hours away. I think that moving away from one's home and experiencing new things is an important part of growing up. I'd be miserable in your situation. I love moving to new places and would hate to live in my hometown. In your case, it sounds like compromise is necessary. If he is willing to move 2 hours away, move 2 hours away. There's no time like the present, as you'll both be looking for jobs after graduation, anyway, so nobody would have to leave a current job to try something new. In a few years, maybe he'll change his mind about living far away and you can move again...or maybe you'll miss home and you'll want to move back. Take a small step now and then reevaluate it in a few years.
3. That's a fair point to consider. How hard has the economy hit your family? Have you talked to them about it? Is the 20K wedding still even possible? The way the market has been lately, I wouldn't be surprised to hear that the 20K they expected to spend on the wedding is now worthless stocks.
Talk to your parents seriously about the economy and find out how they planned to fund the wedding and whether the amount they originally promised is still available. Where is the money coming from, and how much will spending 20K now affect their futures? So many people's retirement funding has been decimated by this economic turn - while our generation has plenty of time to ride this out, I worry constantly about my parents and their friends. I would talk to your parents and tell them that you realize that things have changed since they originally gave you the 20K quote. Tell them that you'd be just as happy with a smaller wedding, particularly if giving you the money would cause them more of a hardship than they originally intended.
Nobody can answer this for you. This is a conversation to have with your parents. Hopefully they will be honest with you about their finances and won't offer what they can't afford to pay. Let them know that you will not be disappointed if they need the money for their retirement rather than the wedding, given this economic change.
4. Stop worrying about this. First off, you won't know for sure whether your career makes you happy until you've worked in it full-time for a few years. Even if you've been the queen of the internship and part-time job in your field, you won't know until you do it full time. Plus, every job is different, even within the same field. I've had jobs I hated in my field and others that I've loved. So give it a chance and if one job doesn't work for you, try another. I'd give a new career 5-10 years (and several different jobs) before deciding it doesn't make me happy. A certain job might suck, coworkers at a certain job might suck...but you might find your dream job doing something slightly different, working with better coworkers, or you might just find that you need more experience to get your dream job...in which case you work towards it. And if all else fails, you can always go back to school and change careers (or do so without going back to school, depending on the field). Your career path is not set-in-stone just because a certain subject is listed on your BA. So give it a chance before you start worrying!

Vice President and Guardian of the Toilet Brush of POOP: People Offended by Offended People