Hi Charlotte5 -
Arno and Annika are the experts on rail passes, so I'd look carefully at the links that Annika has provided . What follows are my best guesses, which are not good enough....especially about extra costs in addition to passes. There may be some for reserving seats, for example. At least, you need to clarify those points.
This link may give you more information about coverage and costs
I can add some comments-
A Swiss travel pass normally does not cover trips outside Switzerland. There are some exceptions for French towns (ski resorts) very close to Geneva, so there may also be some arrangement with Austrian rail services for Lech, Zürs, St. Anton, etc. However, I don't know anything about them, and I expect you will have to pay extra ( in addition to the Swiss rail pass) to return from Lech (and probably Bludenz) as far as the Swiss border on the 14th.
Note that the Arlberg Express bus has different time schedules for different days. Depending on when you arrive at Zürich Airport on Friday the 11th, you may have the choice only of routes B and D (1300 and 1900 departures.)
If I recall correctly, the Swiss Travel Pass comes in 3 day and 5 day versions. Perhaps the "Flex" version would allow 4 days of coverage? But, you will be in Switzerland 5 days on the 14th through the 18th....as Annika indicate, there may be added costs for local transport, etc.
On the 14th -
It is unclear to me if you wish to travel from Lech to Luzern, then onward to Montreux (on the slow scenic route of the Golden Pass route via Zweisimmen) on the 14th. If you do, that will be a long day. On the order of 8 and 1/2 hours on trains, plus up to one hour time to get from Lech to the train line, plus connection time at Luzern. About 10 hours total (or more) .
If you wish to use the Golden Pass route, you must force it to go through an intermediate station on the desired route from Luzern to Montreux. Zweisimmen is the usual choice for the intermediate station in this case. That is because the timetable computer chooses the fastest route by default.
The timetable for the journey from Luzern to Montreux will default to the fastest route, via Bern, which will be a couple of hours shorter, unlesyou force it to go through Zweisimmen.. That faster train trip would allow a few minutes to walk around a bit of Luzern, but would not offer the scenery of the Golden Pass route.
Hope these partial answers help a bit to point you toward areas of concern.