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Not really, I would be more concerned with the 302 redirects on your homepage.

Yeah I believe that regaining the DA back shouldn't be an issue for the old domain. It won't be instant, but shouldn't be a massive amount of time before you get back to where you were previously.

No issue that I can think of, apart from the following:

• Have any links to the old site been updated to the new site? If so can you request for the webmaster to change them back?

Do you have this on any other pages? If so is there a similarity between any of them? Is there anything that you're using that auto generates dates anywhere?

You're not trying to rank your basket page, so I wouldn't personally worry about these.

Without a doubt domain.com/shop. Think about it though from a customer point of view, most languages read from left to right, so with the /shop they subconsciously read your brand name before the shop, however with the shop.domain.com they may just stop when they see shop as that's where they know they need to be.

I would personally approach this from a financial point of view, bring up both pages of content and show your customer the two side by side, then ask them "Why would a customer want to buy from you rather than the other store?" This approach has personally worked for me when all technical avenues have failed, as after all this is the same sort of lines that search engines live by showing the customer the most relevant page.