Yes this is an on-going challenge that is hard to overcome. Shopify does have certain rules around branded products and a handful of our merchants have gotten warnings about these products from Shopify. We do have some documentation but typically not enough so if you get a email from Shopify we recommend simply removing any products they have flagged quickly as to not get in trouble.
I know this is not the greatest answer and not ideal. But this is the best we can do at this time.
We are working to come up with a better long term solution to support you as a merchant.
Very few of our merchants are having any issues so you should be fine.
We cannot give you a reseller agreement. We are continuing to build up that documentation for merchants like yourself but we are not offering those types of specific agreements right now. We have existing agreements with our 100s of suppliers that are extended to you but we cannot provide documentation for you.
In terms of specific copy for your site I would say that you have have a partnership with MerchMixer and we are your official supplier.