How does this work as a jogger?

It works fine as a jogger. The wheel in the front is a foam wheel. If you only have one child, I would say this would be a very bulky jogger. It's very wide and does not fit through normal sized doorways.

Is it safe for just 1 toddler to sit in it? Any weight swift issue?

Yes, it is safe for 1 toddler. All three of my children (5yo, 3yo and 2yo) have ridden in the trailer on their own. You are able to adjust the straps easily to sit the child in the center of the trailer.

Can anyone describe the airflow in the cabin. I would definitely be using it with the screen down to protect my girls from debris.

The air flow is just fine with the screen down. The entire front is mesh. No, rocks and debris don't get inside if the screen is down. When the plastic is in over the screen, don't fasten the front corners down. The kids will get plenty of air. My kids are 8 yrs old and survived using the trailer between 8 months and 4 yrs old.