Is it reasonable to leave Salzburg early morning, spend a seeing Innsbruck (the top 8 to 10 sights during the day) and travel on to Zurich?
I don't know what the 8 or 10 top sights in Innsbruck are, but you could leave at 6:56 am and spend 11 hours in Innsbruck and be in Zurich by 11:20 pm. Or less time in Innsbruck.
There is a direct train from Salzburg to Innsbruck every hour at 56 past the hour taking 1 hour and 50 minutes. There is a direct train from Innsbruck to Zurich every 2 hours at 48 past the odd hours taking 3 1/2 hours. You will need 2 separate tickets.
You will need 2 separate tickets.
This is only true if you travel with "Sparschiene" bargain tickets, which are bound to the selected train(s).
Any regular OeBB ticket for a distance greater than 100 km (60 miles) is valid for 2 days. So you may interrupt your journey as often and as long as you want, provided that you reach your final destination within the two-days time frame.