I think that it's cool that the Portal was made a Mercere rather then a Guernici spell (I wonder if there's any hard feelings with a Terram Gernici or two...?). The involvement of Bonisagus is also nicely hinted by you in the book, but I really like possible motives you voiced here of why he might have downplayed his role!
Whenever I hear the "brings two distant places next to each other" explanation I can never help thinking of good ol' Mage's Correspondance, but I best quit that lest Paradox snatches me away . But basically I like they idea anyway. On a sidenote this was luckily a breakthrough discovery, as I wonder if anyone would ever argue to do the same with e.g. a Rego Herbam ritual (somewhat very Diende or Merinita-like). As is they can just be told to get back to the lab and work half their lifes to make a similar breakthrough if they really really want it.
[EDIT]My Explorer acted up (again!) and I had to restart the computer. I just wanted to add that I do think that the Redcap live exiting adventurous lifes going for and wide, but at the same time I probably don't imagine them to do it to such an extent. There might be plenty of reasons why they all don't just pop here and there for a little time off. Families, danger, inhospitality, duties, other interest or simply a mild censure on the use of the Portals might reduce it a bit. I think of them as great travellers with a long range, but if they all just popped about like modern day globetrotters it would in my opinion became a bit too much.