Advanced Singles and hot weather disruptions: What are our options if something goes wrong?

This Friday my parents and I have advance single tickets from Machynlleth to Euston via Birmingham New Street, leaving at 10:07 from MCN with TfW with the connecting Avanti service leaving New Street at 12:47. These are through tickets bought through TfW's site.

The problem is that TFW are having issues with their trains in the heat, and seem to be running fewer services and/or replacement buses between Machynlleth and Shrewsbury. What I'd like to know is

1. If our departure from Machynlleth is late and we miss the train at New Street, is it just a case of talking to someone/getting on the next Avanti service to Euston? I understand we'll lose our seat reservations but do we need to sort anything before getting on the train?

2. If there's no service at all from Machynlleth (or one leaving very late), would our tickets still be valid if we made our own way to a station on the route (e.g. Shrewsbury for the TfW portion or Birmingham International for the AWC section) and jumped on a service from there?

3. If everything goes to heck and I end up having to give up and drive to that there London, what would our refund rights be?