I think the problem here is that you have a "static" website, and you want a "dynamic" website.
1) How can I keep up with managing some 400 product prices without going to each individual page and manually updating them?
You likely cannot keep up with editing individual pages. You need to look into having some type of product management capabilities.
2) How do I manage stock control on these pages?
Same answer as question 1.
3) Is it possible to change one piece of data which is the same but is also on various different pages from just one area?
You can do this if you are using some type of template, but if you just have a bunch of static pages it is likely that the code was copied and pasted.
I think you are at a crossroad right now. You need to think about what it is you like about what you have and then move forward. If you like the design of the site, then you should show the design to a developer and tell them you want your site to look the same but you want to be able to manage your products. There are open source shopping carts out there like
osCommerce that will give you the product and order management, and you will just need to look for a someone familiar with osCommerce to give it the look you currently have and to port your existing product info. Hope this helps.
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