In my opinion your texting with your H was good - civil and abrupt. Nothing to dissect or find fault with.
I am not sure that talking to the BIL was a good idea. I would love nothing more than to tell all of my in-laws what a cheating liar my H is, but he is still their sibling and son. It would just make things worse - talking to your BIL may have encouraged H to contact you. But, I don't think you want him to contact you because his brother told him it was his obligation. That being said, you can't dwell on it. Now, you take the opportunity to db, be all business about the taxes, light and breezy and you end the interaction first. You can do it. Every opportunity is a learning one. I don't think one mistake will make or break our chances of reconciliation. I don't think it works that way.
Your questions about him moving on and why we really can't answer is because it is mind-reading. The only thing we can control is ourselves and our reactions to our WAS. You have a plan for Saturday, your text with H was good and you knew to ask if it was a good idea to talk to your BIL. So, I think you are on the right track.
M 44 H 46 M 20yrs T 25 yrs S15 S12
ILYBINILWY 7/18/15 Move to MBR 9/8/15 Physical Separation 10/10/15 Suspect A 8/2015 Confirm A 12/27/15 D filed by H 2/2/16