Scotrail Ticket Purchases in September

As stated elsewhere engineering work will not be announced by Network Rail until 8 weeks out.

Thus we don't know if the West Highland Line to Fort William will be affected. However-
1. When in doubt check the Caledonian Sleeper website. They sell tickets one year out as a commercial decision. To do so they get long advance notice from Network Rail of ideas they have, before those become fixed plans at the 8 week point. Those are posted on the Cal Sleeper website. I see no such notice so don't anticipate anything on the West Highland, but until 8 weeks before you can't be sure.
Sometimes also CalMac ferries post advisories on the relevant routes on their website as soon as ideas are released. Again I see nothing.

1. If trains are cancelled rail replacement buses will be provided. When that happens the West Highland is divided into 3 sections with the bus running over the affected section and connecting trains on the other sections. The sections are Mallaig to Fort William, Fort William /Oban to Crianlarich, Crianlarich to Glasgow. Between Fort William and Crianlarich the buses run via Glencoe as there are no roads on the route of the rallway between Tulloch and Bridge of Orchy.

Regarding Question 2- There is a misunderstanding here about the meaning of the word 'Advance'. Full price tickets are available for September dates which carry no seat reservations. You are buying a ticket to travel ahead of time (IN advance), but no guarantee that the trains will run to that schedule and no seat reservation. You can reserve your seat at a later date or run the risk of standing.
But trains can not sell out, so why do that?

Advance Tickets- the cheaper fares- are tied to a specific train. In most, but not all cases, they come with a seat reservation. There are some routes where there is no on train facility to have reservations, but there are advance fares.
Those are only released at the 8 week point (or on routes to Aberdeen or Inverness where LNER or Cross Country also run (XC to Aberdeen only up to 12 weeks) where they take the commercial risk of opening bookings sooner.

The Engineering work you refer to is at Stirling, which is not on the Fort William line.

Question 3- wifi- in the vast majority of cases yes as you have surmised.

Question 4 on m-tickets-
no, you need data to be able to 'activate' the tickets.
Turn your phone into your train ticket with mTickets.

mTickets are the faster way to buy and use train tickets, straight from your mobile, eliminating any need to queue!

Blockquote
Advance, Single and Return tickets can all be bought as mTickets through the ScotRail app, and are available to download instantly. Stored within the app, once you've bought your mTicket simply activate it with a tap turning the image from grey to orange, then scan at the ticket gates to board. Join us in going digital and take the next step towards greener travel.

If you are not going to have data you either need to print at home or arrange to collect from a station machine using an 8 character alpha numeric code you will be e-mailed AND the original credit/debit card to prove your ID.

There is a 3rd way- called a Smartcard. However another forum member has decreed that the smartcard does not work for overseas visitors. Although the evidence for that is inconclusive I lack the energy or will to prove that, or to help people with the exercise. So if you see Smartcards mentioned on Scotrail forget about them.

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The Scotrail app interfaces with Google Wallet (and presumably similar) so once added you don't need any online access. It can be useful in areas with limited connectivity.

@ isn31c ....Thank you for the repost and all the information!
I wish you were writing the website for Scot Rail. Your explanation is comprehensible.

@jgdh....Yes! It's so nice when admissions to places go to the wallet of my phone.