I'd say I'm mostly a 7.
I've not got the clothing extreme of a 7 but definitely have the stair issue of 8. So I'd say that fits.

Lol, I guess mine is actually the same. It fit fine in January. I've not had to use it since covid started and that was 65ish pounds ago.

Lol, I've definitely belly dialed people by accident when I've pulled out my hoodie and found it outgrown since last year.

So mathematically, if you're currently burning all the calories you eat, you should gain 21.5 pounds. I'm basing that on a 16oz jar.

Realistically it'll probably be more like 15, the human body isn't the most efficient machine. Couldn't really predict how you'll gain though, that's all down to genetics.

I don't think there is a set formula here. It's more to do with gaining quickly than with actual size. How well your skin stretches also is a factor. I've known ladies who reached 400 without them and a lady who gave herself a couple from a particularly large buffet trip.

Yeah, that method will definitely get you gaining.
Eating a little past where you feel full will slowly expand your capacity.
Anything you're eating above and beyond your calorie needs will eventually make you fatter. I knew someone who gained about 75 pounds on a vegan diet. You don't have to eat fast food or sweets to gain, but they're an easy source of calorie dense food.

I totally have that issue too. Eventually you can't spread anymore and it fills other space.

If you want to check your scale grab an unopened bag of sugar or flour and test it. They've got a known weight so results should match.

Some scales do allow adjustment, it's uncommon on digital scales but my mom's scale has it. If they have been adjusted it's usually to match the scale at their doctor's office scale.

Mechanical scales can also get off over time as springs wear or if they've been warped or bent. If grandma or someone in the family is near the limit that might have caused the discrepancy. I've personally caused that issue.

The other thing to check is that they're both on a hard, flat floor. If one is sitting on carpet you'll appear lighter.

Lol, been there. Last place I worked I had to enter most the stalls sideways and my belly still brushed the door frame. I was just glad we didn't have the wall mounted toilets other floors did, at least ours didn't make worrying sounds when you sat down.

It's a good start. I don't want to downplay that. It's a good foundation to work from.
However if you're trying to look realistic I'd make a few small adjustments.

You've got a roll bulging out mid back. You either need to smooth that some or pad the lower back a bit more. Back rolls are a thing but its completely isolated right now.

You also need to adjust the padding in the transition from chest to belly. Even in a tightly packed jacket there would be some dip from moobs to belly.

I'd reassess after fixing those two points. Right now it also feels like the belly may be cresting a bit high, but fixing the chest will effect how that looks.

As always with experimental costuming don't make too many changes at once. You're sculpting this look and too many changes at once will make it hard to know what fixed/caused an issue the next time.