I've heard that on some RJ's pax who need an aisle chair to be boarded, can't ride on 1. Also heard that there isn't much room for a whelchair to be transported if they don't fold up. Is t his true? I may have to ride an RJ on my trip to Aalborg, Denmark but also interested in RJ's here in US.

I've heard of no such restriction on Alaska or Horizon air, but I don't suppose every airline configures their aircraft the same way. I'd make sure to talk to the airline directly before booking just to make sure.