1) You are able to set role based  permmissions for modules and pages; so definetly yes.

2) This is not part of the DotNetNuke core though there might be second party enhancements.

3) a.You are able to use the IFrame module to insert your existing asp.net pages transparently.
    b. You might change your asp.net pages to DNN modules for deeper integration. As your pages are part of individual installation where you control your enviroment using mysql just for your modules is no problem

4) You module (~User control) will update your date, it is just hosted inside dnn.

5) It depends on your sites and modules. I have moved modules from IBuySpy to Sharepoint and back to DotNetNuke in short time, but I always tried to seperate the hosting enviroment from my logic using interfaces. Look for the DNN Provider architecture to catch the idea.