Marketpress Multisite Config Decisions

So, on and off for the last three months I have, through trial and error, been trying to figure out the right way to set-up Marketpress to meet my needs in a multisite environment.

Think of the environment I am trying to set up as an "online mall".

I want each vendor to be able to have (limited) autonomy in the look and feel of their sub-site, and to be able to manage products, inventory, pricing, etc. on their sub-site. I want to be able to manage "mall wide" product categories.

Each vendor will be responsible for their own order fulfillment, which means that customer deliveries from different vendors will have different shipping cost calculations. Also, if the site/service is successful, I see it quickly being multi-country/multi-currency.

On the top level store (i.e., the "mall front") I would like to be able to display products available across the whole site based on certain criteria - new products, popular products, all products in a product category, etc.

Vendors will have the option of paying a monthly service fee, or a percentage of sales. The percentage of sales is target at low volume sellers (artisans/hobby producers, of whom there are many in this marketspace). The monthly service fee is for those vendors whose volume is large enough that the percentage of sales no longer makes economic sense.

My questions are in the "which is best to meet my needs" category:

1. Should I activate Marketpress at the Network level or sub-site by sub-site?

2. Should I use a Global Cart (which, if I understand correctly, is the only way to take a percentage from sales) or site-by-site carts (which, if I understand correctly, is the only way to handle the separate shipping and currency requirements)?

3. I've gone back and forth a couple of times between the settings in questions 1 & 2 to see how it affects operations and I must say I have totally confused myself. Is there something else I am missing in setting up MarketPress/Multi-site to meet my needs?

(BTW, I would love to RTFM, but I haven't found a manual, and haven't had much luck finding information that is "business decision" related.)

Thanks in advance for any help you can provide.

John Hollier