Two related questions
1. Traveling from Innsbruck to Pontesina, CH. I have a SR Pass. Is it best to buy a ticket just to the border and then use SR Pass for onward journey?
2. Can I use SR Pass from Zurich back to Munich or will I need to buy a new ticket at the German border?
The Swiss Travel Pass is only good up to the last stop before the border
This is the area of validity
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelsystem.pdf?sfvrsn=7788603a_36
In all cases the Swiss Pass is only valid on Swiss rail lines.
However note the caveat "Swiss rail lines", sometimes Swiss rail lines cross the border, in which case the pass is valid, in some cases "foreign" lines run between two Swiss destnations, in which case the pass is not valid.
Exactly what the Pass does and does not cover is defined in the Pass Map here: https://www.sbb.ch/content/dam/sbb/de/infotexte/uebersichtskarte-sts.pdf
Innsbruck to Pontresina. Pass is valid from Buchs SG (see map). I would go to ÖBB (Austrian Railways), and price out a ticket (A) Innsbruck to Buchs SG, and (B) Innsbruck to Pontresina. ÖBB does some great offers, it may be cheaper to buy a ticket to Pontresina than to use a day of your pass.
Zürich to Munich. Again pass is valid to St. Margrethen. Price out both Zürich - Munich and St. Margrethen - Munich on the DB (German Railways) website. It may be cheaper just to buy a ticket from Zürich and save a pass day.
- The route to Munich is being re-built. Most of the entries shown on the DB website for this route are rail replacement buses, running non-stop. Make sure you know what you are buying a ticket for.
@Jazz
The Swiss Travel Pass is only good up to the last stop before the border.
Sometimes it is the first stop after the border, for example Tirano.