Ahh-Some questions

From my experience, Ahhsome works better when warm, and even in 38C spa water, it's slow to dissolve. I would dissolve the gel in a bucket of hot water first. It should work in the spa at well water temp when well dissolved and evenly spread in the water. Ensure you fill the spa to at least the fill line if not higher, open all air jets, and remove the filters. If your filters are brand new, keep them out of water. If filters are used, keep them in the footwell.

I would only stabilize the chemistry enough to not create scale on the heater (ensure pH doesnt get above 7.8), and also enough to not leach metals out of the heater (if your source water is soft, you'll need to add calcium and also ensure pH doesn't go below 7.2). I would raise chlorine to 20ppm using liquid pool chlorine, then add more chlorine every hour.

If the Ahhsome stops foaming and stops producing gunk, dissolve another teaspoon in a bucket of hot water and add it. If more gunk comes out, add more chlorine. The instructions say 30 minutes, but my first run took a few hours before the gunk stopped (I was scooping and cleaning as it went along).

Anything else you need to know to fill would be in the owner's manual. Typically you shouldn't fill with hard water unless you adjust hardness upfront, then balance TA, then balance pH, then add chlorine. Do not predissolve calcium chloride in a bucket or glass jar, it's an exothermic reaction and gets real hot. Also do not add TA increaser/baking soda within 24 hours of adding the calcium, they bind together and cloud up the water, giving you less of each in your water testing. Just use PoolMath, test your fill water, then enter those numbers to pre-calculate how much of everything you need in total.

20 ppm fc is a bit high considering there will be no cya. I would do enough dichlor to get to 10ppm. That should be sufficient.
take its ph effects into consideration.

Last time I didn't have time to change the water right away-- and I left it in and running for a week. The Ahh-some did a much better job running that long than when I did it only an hour or two before the water change the time before (I have an USB endoscope and I do use it..).... I'd agree to get the water warm and let it work that way. On a brand new tub I would probably not bother.. I didn't and didn't have issues. If you are worried.. change the water early.. maybe after two months instead of 3-4....

So for softened water... they are not wrong... but... measure your calcium hardness and possibly mix the hard water and soft water together. On a 250-1000 gallon stand alone tub you can even do crazy stuff like put 20 gallons of RO in (or at least in my 250) to get the fill water to be more ideal-- stuff you could never afford to do in a 15K gallon plus pool...

But if you put in no carbonate? You'll be adding it later and you could mess up the heater core.