I will implement option 1 above (ctrl-click to select and delete multiple groups).

Details:

1) Behaviour if you only select a single group will be the same as it currently is.
2) The confirm page will list all groups being deleted as above.
3) Using JS, the 'edit' button will be greyed out if you select more than one group.
4) The edit script will be modified to give an error in case you select more than one group (with JS disabled).
5) The help file for this screen will be slightly altered to mention that you can use multiple select to delete groups.

This will be done for 1.9.4 and 2.0.

group/clientlib.js
group/delete.php
group/group.php
group/index.php
lang/en_utf8/group.php

Here are the details on the change:

1) Some other minor bugs are fixed as part of the change, including:

• initially-specified group name (or any selected group name when non-AJAX) was not shown under 'Members by' heading.
• when in non-AJAX mode some buttons were disabled until you clicked 'show group members'; this was inappropriate and meant you had to do an extra pageload/click for no good reason.

2) I added the new error strings 'errorselectone', 'errorselectsome' in groups.php langfile: 'Please select a single group' and 'Please select one or more groups'. [These only apply when not using AJAX so that you can click a button when you haven't got the right things selected. The system previously gave unhelpful errors in this situation.]

3) You can still access both changed scripts with the same parameters to refer to groups: ?group=N in the case of index.php and ?id=N in the case of delete.php. I made sure to retain these as working, because they are/may be linked from elsewhere. However in the case of index.php you can also pass in group[]=1&group[]=2 etc (ie php array from the <select>), and in the case of group.php [used for delete] it actually redirects to a new delete.php?courseid=N&groups=1,2,3 etc (PARAM_SEQUENCE)

4) I had to add one non-error language string. This language string 'deletegroupsconfirm' is only used if you select multiple groups. If it is not permitted to add non-error language strings to 1.9, this string could be deleted (replaced with nothing and just the heading 'Delete group' to clue the users in as to what yes/no means).

5) There isn't actually a helpfile for groups, so I didn't edit it

I tested mainly on core MOODLE_19 and briefly on HEAD (which required changes for new DB syntax).