1. Workflow forms generated by K2 blackpoint is done based on SharePoint best practices, therefore should security not be an issue with the forms as it is no different than OOB workflow based forms
2. There should not be any noticeable impact
3. With the auto generated forms you have the ability to select fields that should be available on the form and whether they should be read only, required or optional. Based on your requirements described here my 1st choice would be to make use of K2 smartforms but that is a K2 blackpearl add-on only (but it’s worth mentioning). InfoPath-based workflow would be a better option (than the auto generated forms) to get the required functionality into a form.
4. Depending who you ask this question to will depend on the type of answer you get, this also depend heavily on the tools you have available.