NO-as such not, but that very much depends on what other trips you intend to take. But in general your research is right, I ´am afraid´ in neat /en/.
Best buy for short stay visitors wanting more as just the city and staying a bit longer is a 7-day all zones ticket for 53,20 (may start any day-just buy at any staffed station), so this may also include the in Tirol sectors of your trip in/out. I used this myself some years ago for some 5-6 days then&there. Good value-also includes a direct express bus to Ost-Tirol, and the transit Germany railline to Reutte.
Tickets to/fro INNsbruck have as MINimum the indeed higher fare that applies for that city-but it does not count as double zone.
Prices are also much lower-nearly 50% for ANY senior from 65, no further cards needed for that.
You also did not clearly specifiy what other BUSlines you want-as there are just very, very few connecting to that Stubaitalline.
For just city Innsbruck+return on that line, it would appear to me then best buy is 24 hr INNsbruck city for 7€+ additional 2 singles out+return to ride that line. From the german Tariff folder as presented on their website (VVTirol) it does indeed NOT show any daytickets as ´return´ as indeed most-but not all, Austrian province/Land organised VerkehrsVerbund traffic areas offer.
Just 1 remark: in general the ´free for children´ is quite common and in most cases does not really mark up the price as it would be for just 1 or 2 adults on their own. One probably also does not want an endless list of any possible combo of small groups or families and just slightly differing prices for it all.
