Base in Val Gardena

It would be an issue for me. Take a look at the most recent five years' worth day-by-day temperature statistics on timeanddate.com and see what you think. It you can find a place at altitude, you might not need a/c.

You say ValGardena in your title, but then name Cornaiano which is nowhere near Val Gardena. The lower elevations towns (like Bolzano) are hot and humid in summer and one would want AC, but I don’t know about September. If you stay in one of the Val Gardena villages (Ortesei, Santa Cristina, Selva, etc.), which are at higher elevation, AC should not be needed in September.

Ortisei would be my choice. Selva and Santa Cristina are also nice but smaller.
At night you will need a sweater not air conditioning upthere.
Cornaiano is down in the Adige Valley and not in Val Gardena. It wouldn’t be my choice for the area. I would stay in Val Gardena proper. The Adige Valley gets hot for sure. It’s considerably lower in elevation.

If you want a great base in Val Gardena ; choose a town actually part of Val Gardena.
Ortisei would get my vote, beautiful small walkable town ; you do not need AC though you may have to pay for parking.

Would not leave the car parked and take the bus. Bus route options in Sept through the Dolomites will not be great and a car is highly preferred, traffic and parking are rarely an issue in this region.

As I said Ortisei is a great town. Small, walkable but plenty to do in town and in the surrounding areas. Never had an issue with parking while in town. Most hotels have parking and from your hotel you are likely to be able to walk to the city center where parking may be available only in parking garages for a fee since the area around the main drag (Strada Rezia) is pedestrians only. Last time I was there I stayed at Villa Emilia hotel, with a nice parking lot. Not far from the town center. To visit the Passes and the other areas, I would drive since you plan to have a car. Much more efficient than buses, that way you are not subject to bus schedules etc.

Thanks for the clarification. An area new to me. I was staying further south to take a advantage of the warmer weather..clearer skies might of thought. And then planning on day trips to Ortisei,etc. I had read also that Cornaiano had good bus connections further north so we could avoid driving. I will look at Ortisei. Drawn to the area because of my German/Austrian heritage. If we are not into multiple cable car lifts and and hiking is this the wrong area for us? The visual alone has drawn me in.

To add to that..if my base was in Cornaiano or a better town in that area..I could do the wine road, Merano, visit Ortisei, maybe Bolzano...whatever combination of things we decide to put together. If I am in Ortisei, then my focal is of course ambiance...already found a possible hotel...and lifts and hiking? Further educate me on this .

There are some good advice points on this related rick steves link: https://community.ricksteves.com/travel-forum/italy/dolomites-visit

Here is where I recommend: https://www.stuacatores.com/en/

We go every Sept (except 2020!) and the weather has, mostly, been delightful in Ortisei. No A/C but plenty of day where we hiked in short-sleeved shirts. We take layers and often leave i the morning with a tee shirt, fleece, and jacket but return in short sleeves.

Bus route options in Sept through the Dolomites will not be great and a car is highly preferred

As to transport, yes, some busses cease after the 3rd Sunday, especially those between valleys. But you can enjoy everything in the Val Gardena via bus and lift. If you have a car, most lodging has parking and you can use that when you dare to venture over the high passes. If you only have a few nights, you will may not want to leave the Val Gardena at all. Depends on your activities.

If you like spectacular mountains, stay in the Val Gardena.
If you like wineries and wine, stay in the Val d'Adige.

Well I am going to look into both. If any one knows of a nice property in Val d Adige feel free to pass it on. Seems the one I picked out in Cornaiano has no AC which I now know I need. In the meantime will look at Villa Emilia. I found one that is right above a bakery and serves strudel for breakfast..right up my alley :-)