We've seen people with this type of request before, but basically everything boils down to trust. This is of course simple if they have admin access or access to WSI, but you won't want to provide all of your partners with complete access to your store. You could however provide them with the necessary syntax to create an excel document or WSI document which they could then send to you which you could then upload yourself, which is probably the best option but will require time on your part to review and upload. The only other option is to make modifications to allow them to upload only inventory in the manner you see fit.
As far as the shipping, you can get close to what you want by setting your shipping rates calculation method to Use Individual Item Shipping Costs and by setting the appconfig FilterOutShippingMethodsThatHave0Cost to true then only setting the price on the items for the shipping methods you want to offer for that item (leave the price as 0.00 for the other methods that you don't want to be used for that product), but that seriously limits the flexibility of your shipping. You won't be able to order items from both partners at the same time without seeing all of the available shipping methods even though item A's shipping methods might not apply to item B and vice-versa. The best option here would be to make modifications to create a way of mapping shipping methods to partners and partners to items, and then modify our shipping logic to display only the correct shipping methods for the items in the cart.
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