Reusable user control (ascx) design for multiple web application
Is there any better elegent ways to solve my problem?


You are going down a horrible path messing with all these user controls.  ASP.NET MVC is much better for dealing with reusable visual parts, and everything happens on the request level so you don't have to worry about horrible event ordering.

Can't use MVC?  Forget the complex events of user controls entirely.  Put no code in user controls.  If you need the data from a textbox in a control for example, pull it directly from the post variables (wrap this in a class if you need to)

Make your goal to deal with requests in a SINGLE, centralized point.  It's the MVC way, it's the MVP way, it's the MVVM way.

Need ajax?  scrap ajax controls and hit a web service;  throw away the ajax toolkit.

I actually wish I had an old crufty webforms app that I could re-work this on.  I would completely bail on the failure that is asp.net lifecycle