Red Guy, here's some reality straight from a Mesaba guy. Neither you or your friend will get hired at XJ as a CA anytime soon. They have been very clear about wanting street CA's to have previous 121 experience. I don't mean that to sound demeaning on your 135 experience (I was a former freighter myself) just the reality of the situation.
If I were you, I would apply for a FO position at XJ as soon as possible and cross your fingers for the Saab. You'll then be paid and trained as an FO which will last a couple of months.
I would strongly consider flying the line as an FO for at least a couple more months before taking the upgrade. 121 is very different from 135. During your first year at XJ you are on probation which means they can fire you for basically any reason. Like you said, you have a long career ahead of you, and I'd hate to see it get ruined so early. But that's just me, do as you like.
CA training in the Saab is about the same length as your FO training would be. You don't receive CA pay until you are completely finished with CA training (your fed ride compete).
Hope this helps!
-IHM