Go here!: Please let us mix and match different products when ordering
Personally, I’ve never ordered the granola, plus I am in the UK… so I can’t be sure what the issue is here, but there has been mention of the granola possibly being discontinued so perhaps that’s why you can’t subscribe to it?
As for adding a scoop and ensuring your order contains everything you need, it’s probably easiest to email them: team@huel.com and they may be able to manually modify your basket.
You can only add one ‘single’ RTD. You should be able to order a case though (?) Again - ask them when you email them. There could be an issue with delivery eg with the weight of a case, specifically in your area / country
The samples are just samples and you only get 3 grams. If you use a strong flavoured base powder, or more than 100g, or more than 400ml water then you need more than 3g of boost. Personally, with the samples I just make up a small shake to try out the flavour. If I like the flavour I buy a full bag and add closer to 5 or 6 grams per shake for a good flavour.
Search the forum and you will see lots of mention of this and lots of solutions. Start off with just one shake a day and build up, let your body get used to the extra fibre. Drink plenty water.
Yes. With v3 they changed the portion size to 400 calories (100g / 2 x 50g scoops) as this is a much simpler measurement. So the bags now contain a greater number of meals, because the meals are smaller! The weight of the bag is the same though.
Phew! That was a lot of questions in one post 
Did I miss anything?!
Have a read of the forum and you’ll find lots of answers (and lots of bad jokes, and a few pictures of cats).
But, for your ordering issues, definitely contact Huel directly by email (or live chat if you get that function in your country?)