I know it's just my opinion, but ditch the crowd calendar projection things...they're just wild guesses and have no true accuracy value. If you want a good spot for HEA on Dec 20th, I'd honestly suggest being at least 90 minutes early. We were there this past Saturday night and walked down Main Street 2 hours before HEA and there were people already lining the hub. We're going back to MK tonight, I can report back if you'd like.
Yes, if you're going to be in front of the Castle right before HEA, then you can expect wall to wall people on Main Street. Those pictures don't surprise me in the least. If you were going to try to walk down the actual street during that time, I would consider that foolish. You need to use the sidewalks and/or cut through the stores. It was definitely be slow going, very slow going at times, but you will be able to get through. They do a good job of keeping the sidewalk lanes open.
Frankly, Main St looks like that during some summer nights right before HEA (or formerly Wishes). Adding the holidays just makes it more busy. The actual street itself is a sea of people, use the dedicated walking lanes.