I haven't seen any solid evidence as to when Parsons might have reached Monterrey, or exactly how he got there, but I don't believe he was ever part of Shelby's column. In fact none of the primary sources I've seen in regards to Shelby's march from San Antonio to Monterrey mention him. According to one source I've seen, Parsons and his entourage went to Monterrey via Matamoros, not Eagle Pass, which if true Parsons may have even reached Monterrey prior to Shelby's column. I've been trying to piece together Parsons journey and murder for a while now but its been extremely hard to trace exactly where he was and when. I'm almost positive though that he never traveled with Shelby's command even though he sometimes gets lumped together with them in some of the secondary literature.

Good luck with your book, I'm looking forward to it!

Regards,
Matt