Wow, some of these are pretty technical questions:
This is tough to say. I have seen other people on the forums saying that they worked with Shopify support to work this out.
Sales should, in theory increase if you reach a wider audience but this also requires much stronger marketing and larger ad spend. You also may find that the barriers are too high in terms of international shipping and taxes.
To my knowledge, there is no good way to sell your POD items on another business website unless they have a specific integration I’m not aware of. The best option may be to have them link to your site with an affiliate link.
Same as question 3 except that POS is even more limited than the website. This is probably not doable, if I’m understanding the ask.