1) I have faced the same issue in my Lab ISE. I used a workaround of using the "Test Portal URL" for the sponsor portal. But I agree, this is a pain to work with in a production environment. Looks like there is a bug filed for this issue that should be fixed in ISE 2.2 version (due sometime within the next month or so).
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCuz65011/?reffering_site=dumpcr
(BTW the workaround provided the bug did not work for me)
2) What timezone does the Guest reside in? According to documentation, the Guest access time should be configured with regarding to the Guest user timezone:
Guest access times are based on the Guest Location's time zone. A Guest user may not be able to login if the Guest Location's time zone doesn't match the system time zone. In this case, the Guest user may get an "Authentication Failed" error. You might see the "Guest active time period not yet started" error message in the debug report. As a workaround, you can adjust the Guest access start time to match the local time zone of the Guest user by using the Manage Accounts option.