An outward portion of an anytime return allows travel for 5 days from the start date, and can be used for unlimited breaks of journey. They can be pricey though. In some cases outward portions of off-peak returns can be used for breaks off journey, although travel must be completed within a day, unless this is impossible.
Are you sure about that 5 days thing? I've never heard that before.
My understanding was that with anytime returns, the outward portion has to be used on the date on the ticket. The only exception being an overnight break of journey in which case you have to continue before 12 midday the next day.
This was one of the changes sneaked in with the "simplification" nonsense: previously an open return was valid for the whole month outward and return.
As far as the original question is concerned: you can definitely break your journey as many times as you like on the return portion if you buy either an anytime or off-peak return.
The good news is that the permitted routes from Inverness are quite generous so you can make quite a lot of diversions off the main lines. I have taken advantage of this many times myself! You need to get your hands on the appropriate map in the National Routeing Guide. If memory serves me correctly, it's map AS.