Quanto tempo de antecedência preciso para reservar bilhetes na West Highland Line na Escócia para uma viagem de várias partes?

The only train tickets in the UK that must be booked advance are for overnight sleeper trains, and Eurostar. All other trains do not have mandatory reservations. You can buy your tickets at a station, and take a seat if one is available, or even stand if it's particularly busy (though I wouldn't have thought these trains would normally be THAT busy).

Sometimes buying in advance can be cheaper, there's no doubt about that. But that generally only applies to longer journeys — for the two trips you mentioned advance tickets are not available, so anything bought in advance would be exactly the same ticket and price that you could get at the station on the day.

In addition, tickets bought from the station are usually quite relaxed in terms of restrictions. For instance, in this case you would only need to buy a return (what Americans would call "round trip") ticket from Glasgow to Oban; you could then break your journey at Ardlui and resume it at Crianlarich. You would not need to buy separate tickets for each leg of your journey. Such a return ticket would custo £ 40; the outward portion (Glasgow to Oban) would be valid for one day (with a few special cases; I'm simplifying here) and the return portion would be valid for coming back over the next month (including breaking your journey over multiple days if you so wish). The only main restriction worth mentioning is that you can't do any part of your journey twice — that is, you can't walk back along the route you've already taken by rail and then repeat it. It also can't be used before 09:16 on weekdays (but there's a slightly more expensive ticket which can and is also valid for five days on the outward portion).

The other thing to mention is that day return tickets are also available — as the name implies these are only really suitable for day trips, so avoid these; you want the plain old retorno tickets (Off-peak Return and Anytime Return).

Depending on how much rail travel you plan, you may find it convenient to get one of the Britrail passes for Scotland. The pass has to be bought from outside the UK, and is only valid for people who are not UK residents. However, if the trip you describe is your only rail travel, it would be much, much cheaper to buy a return ticket.

Once you pick a travel day, the pass turns into a go-anywhere ticket for the area in which it is valid. That allows a lot of flexibility. For example, if the weather is so bad that running is not practical on the first day, you can go back to Glasgow, do some more rail touring and indoor activities, and then take a train to Oban later in the day.