Thanks for the debugging. What your warning say is that 1) the host sidebar you're trying to merge with does not exist (in the database) or 2) the host sidebar is simply not an array (which it really ought to be).
I have been able to reproduce your bug under the following circumstances:
1. Clear all instances of widgets and sidebars in the database
2. Create new content aware sidebar that merges with host (sidebar-1) from the top
This will give the following notice and warning:
Notice: Undefined index: sidebar-1 in ... on line 410
Warning: array_merge() [function.array-merge]: Argument #2 is not an array in ... on line 410
However, this error will dissappear when you add a widget to any of your sidebars, as this would create an instance in the database again. This shouldn't be the cause of your problem.
1. Create a content aware sidebar (1) with some random host
2. Create another sidebar with (1) as a host, merging with it from the top.
3. Remove sidebar (1)
4. Remove all inactive widgets belonging to (1) and add a widget to a random sidebar (to update the database)
This will give the same errors as above, only with another sidebar id. It is more likely that this might be your issue. However, the error is only reproduced ca. 1/2 times and I can't really see how this is what is causing your problem as there - as far as I can see - is no difference in selecting Events or Event Tags/Categories for the specific sidebar.
IF this is the problem, I suggest that you check that your host sidebar exist and that you drag and drop some widgets around in order to update the databsae correctly.
About your other question, I have thought about adding the selected rules to the overview before, but I haven't found out how to show them in a clear way yet. It is definitely a good suggestion though.