Re: v4 roadmap and upgrade to v5
You can edit templates but not php files. However editing templates while allowed is not a good thing to do unless it is done via custom module.
To put it simple - any change you want to make to the cart has to be done via module. This ensures whatever you are changing stays even when you upgrade cart or modules you are modifying. Also when you have it as a module you can turn it on/off simply by enable/disable the module.
Any other way of making changes to XC5 should be consider temporary and unsafe. Even the built-in template modifier won't do you much good if you need to edit template multiple times or replace a template...
Directly editing files is completely out of the questions since next upgrade of cart/modules downloads and replaces the whole not just applying changes like the XC4 upgrades do.