I used to work at Volusion and have decent experience with other platforms out there and do lots of seo for ecommerce.
A. Forgot about the CMS. Evaluate the ecommerce platform for its ecommerce, not for making blog posts. For content postings and stuff, make it a subdomain, or make the main site the CMS and the ecommerce a subdomain. (shop.stuffonsale.com)
B. For Great seo for ecommerce you need to run your own fully customizable platform. Volusion has very limeted SEO functions. They are decent for basic sites, but if you want to get into awesome customized category, product, plus other hierarchies then definitely dont go with a hosted cart solution. Companies like Shopfiy are great for selling like 50 t shirt SKUS, but have very limited flexibilty in how the site works.
C. Hosting carts main benefit is you dont have to worry about uptime & tech changes. Hopefully you have someone technical on your team or you have access to a great developer. With ecommerce there is a very HIGH cost of changing platforms at some point in time. You will have hundreds of indexed pages at a given URL structure. If you change then the URL structure with all platforms is different so you would have a big drop in traffic as you start to 301 all the pages.
D. Most platforms now will allow custom design changes at varying levels. Most support custom CSS changes and template changes. Hosted carts will always still be less flexible than a self hosted cart because on self hosted you can truly edit any file that is part of the shopping site. Hosted carts you are limited to what they give you access to.
E. Your last couple questions are kinda vague. You have to have a dynamic ecommerce site, static shopping sites are a headache to manage unless you are talking about just a couple SKUs.
F. I would recommend you find a developer or another person that has created several successful ecommerce sites using different platforms.
Maybe I should write a blog post on how to pick a platform for shopping.
