What's it like dating as a witness?
To some extent it depends on the congregation and your family. I've known plenty that were disastrous, as you've described. And if you so much as touch her boob, you could find yourself being harassed by the BoE. And if you don't touch her boob, but someone else says they saw you touch her boob, you could find yourself being harassed by the BoE.
Would it be worth it to stay home a few more years?
Not in my opinion. The way your plans sound, you need to keep moving ahead. If this relationship doesn't work out, those years will be wasted. If this relationship does work out, you will have to be prepared to "stay home" forever, give up all the other things you want and live as someone you really aren't, and you will never get to know any of the other "worldly" women that would love you for who you really are.
And how's strong is the pressure to get married?
It won't just be pressure that your relationship will be chaperoned and headed to a wedding, you will be pressured to be a MS and "do more" in the congregation.
There's a girl in my congregation who's the same age as me and I really like her. Not like a crush that you have for a month or two and then move on, I've liked her since I met her which was 3+ years ago.
I don't want to come across as belittling your feelings, and I have known plenty of couples that have been together since preteenage. But in my experience, although I had plenty of heartbreak with "sisters", I'm now very glad that I dodged every one of those bullets!