These were also emergency works, so even if there may be any local, private alternatives (a quick glance at a map or two doesn't really suggest anything feasible to me), I suggest there may not have been enough time to check & open them.
This is apparently one of the longest diversions in the UK - I'm not surprised. Unfortunately, part of the tradeoff for experiencing the beauty and unspoilt nature of much of Scotland, or working in areas surrounded by swathes of uninhabited countryside, will always be the usually more limited contingency options for when the transport links go a bit wrong!
Rural areas in Southern England also have problems, whilst nowhere near as extreme - a closure of Betchworth Crossing (B2032) can result in an official diversion of over 10 miles, including over Reigate LC (A217), adding considerable pressure on existing queues during peak times. Compared to the issues at Garve, though, that's nothing.