The problem with SinglePost control is fixed and you will be able to test it in the upcoming beta 2 version of Sitefinity, scheduled for the end of this week.
Unfortunately, I am not able to reproduce your other problem with Sitefinity3.0 beta1 build. Actually, this problem occurs in the alpha version of Sitefinity but in the new build, the design time exceptions for controls should be caught by the control and not the page and the ability to delete or edit the control should appear. To reproduce your problem I have followed these steps:
1. Created user control called TestControl.aspx;
2. Added it to the Web.config using this statement:
<add name="TestControl" section="User Controls" url="~/UserControls/TestControl.ascx"/>
3. Added it from Sitefinity in edit mode. When click on preview everything was fine.
4. Renamed the control to ControlTest. Renamed the web.config statement to this:
<add name="ControlTest" section="User Controls" url="~/UserControls/ControlTest.ascx"/>
5. Went to page edit in Sitefinity and the control threw this error: The file '/Sitefinity3/UserControls/TestControl.ascx' does not exist.
I was able delete it and add the right version. Then I clicked Preview and everything was fine again.
Please, let me know if your problem was the same or I am missing something.
All the best,