Oooh - and now with the DART Orange line connected to Dallas, you can get to the Amtrak Dallas station by train (with a transfer to the Red or Blue line at West End)!
Lessee here. First of all, 421 runs (from Dallas) on Mondays, Wednesdays and Saturdays, not Fridays. That's why you're getting 22 to St. Louis, etc on Thursdays and Fridays.
But even if your connection in Dallas were on a Wednesday, you'd have to get on the Orange line in DFW no later than 10:15. If you miss that, you're hosed (not really - so long as you can get a cab by 10:45, you should be fine).
I might would actually prefer (and it would be safer, time wise) to take the free shuttle to CenterPort and ride the TRE to Ft. Worth and pick up the Eagle there. Latest TRE that leaves CentrePort is 12:46 PM and gets to Ft Worth Intermodal at 1:22 with 50 minutes to spare, but no telling how long it takes to get from the terminals to the train station - only that they have departures every 15 minutes.
The biggest issue is your Thursday connection. You'll have to wait 'til Saturday until you can take the train. The next issue is whether your arrival time is early enough and can be assured to catch a train from the DFW area. Hate to be a downer on Amtrak, but you'll be better off going out the next day if you miss your connection. If you do miss your connection, and you have to lay over, take the Orange line to the West End and have a fun evening!