I'll try to answer all of your questions:

I'm slightly confused as the recipe doesn't call for plain flour. It calls for all-purpose flour, or bread/strong flour if you are in the UK. We don't have plain flour or self-raising flour here in Canada. We have all-purpose flour and it is somewhere between the UK's plain flour and strong/bread flour. We can bake bread with all purpose flour as well as make cakes etc. That's why it's called all-purpose. In the UK you need to use different flours for those different purposes because the protein content of your flours is different. Plain flour has a low protein content. Much lower than all-purpose flour. And that's why all-purpose flour is suitable for this recipe and plain flour isn't.

For this recipe to work well you will need to use strong flour, sometimes called bread flour, as per the recipe. That's because it has a higher protein content than plain flour or self-raising flour and is more similar to our all-purpose flour than the others. Self-raising flour wouldn't work well for you. If you used it you would end up using way too much raising agent, or not enough if you omitted the baking powder in the recipe to compensate for the self-raising flour, and also the protein content of self-raising flour is too low so your dough wouldn't work out well.

With regards a tablespoon, that's the same in the UK, Canada and the US. It's 15 mls. All measuring spoons in those countries are the same.

9 tablespoons of flour is what you need for this recipe and it will fit well in an average-sized drinking mug. You don't need an extra-large one. This isn't like other mug cake recipes. It's a sweet roll made with dough so it doesn't rise like a sponge does. If you only use 4 or 5 tablespoons of flour you won't have enough dough to roll it up properly and hold the strawberries.

There are no cup measurements in this recipe, but in my recipes that do involve cup measurements, I always include the English measurements too. You'll find grams and mls anywhere that needs them. There are never just cups listed, and a tablespoon or a teaspoon is a standard measurement for writing recipes in the UK, the rest of Europe, US and Canada.

I am English and live in Canada, and in normal times spend a lot of time in the US because I live right by the border, so I'm familiar with ingredients/measures in all of those countries. The majority of my audience are American and Canadian but because I'm English I always include English alternatives and measurements.

Rest assured that all of my recipes are tested thoroughly by myself and a team of recipe testers, a couple of whom are in the UK/Scotland, before they are ever published here, so if you follow them exactly as written they should work well.
I hope that helps!

I'm really pleased you were able to adapt it to suit and get a great result Jane!

I would imagine in the Instant Pot it would end up soggy and unpleasant and I have no experience with air fryers. I wouldn't imagine it would cook through very well in one of them but I could be wrong. It would probably work ok in the oven in an oven proof dish, but I haven't tried it so am not sure how long it would take.

I don't really have much experience with gluten-free flour but I believe some people have used an all purpose gluten-free flour to make the cinnamon roll with good results. I doubt very much if rice flour would work on it's own. It would need to be blended with something else. Possibly oat flour but I've never tried it so can't say for sure. You can certainly decrease or omit the salt though if you want to.