Hello Ollie, Ben here - your Shopify Guru!
Seems your having some issue here some of which are easily fixed. First and foremost you have products with errors. These will not be displayed until you fix them. You can find all these errors listed in the account section under Facebook in your Shopify admin.
For the random products not appearing and needing to be saved to update them, usually if you just toggle the visibility for Facebook off save the product. Then toggle it on and save the product this will correct any issue. Alternatively you can manually force Facebook to re-synchronize your collections this usually fixes any issue with products our of place or collections being duplicated.
Keep in mind it can take up to 17 minutes for everything to re-synchronize.
2. The method above should help solve this problem.
3. Correct you can only purchase one item at a time because you have the account set to push your customers through the Shopify Checkout. Your are not eligle to use Facebook's checkout because:
- Your shop must use US dollars
- Your shop must ship to the United States
You also do not have a webstore front without that there is no cart for the customer to continue Shopping on your online store. This is why one item is taken straight to the checkout.
4. This also falls along the lines that you do not have a webstore front, we cannot direct the customer back to a product page to choose the variant of that product which is causing this issue. For this issue please send us an email at support@shopify.com explaining you have to variant selection and include what products this effects and we will look into this deeper.
I hope this helps! Feel free to reach out if you have any further questions :)
Cheers,
Ben C - Shopify Guru
Support@Shopify.com