I was once refused to buy a ticket for 2 months and 14 days (or something like that) because the guy only thought that you could buy tickets for a month or 3 months

Went to the station down the road (thankfully , not too far away in my case) and bought one with no problem. He possibly made the same mistake that MadCommuter made, but there's really no excuse for such a minor error on such a significant commuter line.
The Scotrail staff in the old SPT area are hit and miss. Most of them know all they need to know, and a bit more to boot; but a lot seem to only know how to issue "a return to Glasgow".
MadCommuter, the
season ticket calculator is a handy tool.
Having a longer period season ticket is excellent. Even if it's not necessarily providing a fantastic saving for your purposes, having an annual ticket is extremely handy in that you only have to buy a ticket once a year and can pop in and out of Glasgow as often as you like for leisure purposes (in addition to your commuting), effectively for free. It's a huge initial cost (in your case, £744 pa)and I don't think Scotrail have a Direct Debit option at the moment, like some TOCs do. I think that puts a lot of people off.