1. I do not want to use default field customization in Jira, becuase its limitation.
2. Several plugins cope with this problem, using Validators (for example: JSU, Jira Misc Custom Fields), but the Required Field validation is activated only after the user try to create an issue.
3. How to present in Create screen/transition Required fields in advanced? [Jira Behaviours Plugin is supposed to do it, but it's not ready for v. 6.2]
You've just explicitly ruled out all the best ways to do it, so I'm afraid you're stuck.
The best way to do it is use field configurations as intended.
The second best way is to use validators, although you could look into combining them with some (usually ugly, and decoupled from the validator) hacks to place informative text on the page to warn users that the fields are required.
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