Checkbox on multi select for select all, check exclusivity on calendar

[Justin A]

I have a JIRA issue collector form with a multi-select list for the issue type. The issue types (there are four) represent physical areas within a building that can be reserved for use. The form has a Start date/time, and a Stop date/time.

Currently we don't have any checking (validator?) on double booking because generally each of the areas can accommodate multiple reservations (and because we haven't looked into this yet, and didn't want to be strict on this aspect). We just manually contact the person to qualify their need and adjust the event as needed.

But I'd like to add a checkbox on the from, and label it "Special Event" (special circumstance style of booking--for this exercise effectively means 'whole building'), and have it do two things:

1. Select all four areas in the multi-select (reserving all spaces)--if user then deselects one of the areas then the checkbox would get unchecked (just like any standard select-all checkbox)
2. Check the calendar for no collisions (only for whole-building reservations) on date/time of any existing reservation, either dynamically (via AJAX?) or upon clicking Submit button.

I'm using JIRA's JavaScript issue collector script to place the form on a remote webpage, so for #2 ideally the form would check the JIRA database and not submit the form if any area+time period overlap, perhaps popping up a message listing the exact collisions/overlaps, so that the submitter can adjust their time and reattempt to submit.

(I also have FullCalendar.io on this remote page, and I fetch the calendar data via ajax and show the four area reservations on a calendar; but users can't be trusted to refer to the calendar and not attempt to double book).

Can anyone steer me to accomplish 1 and/or 2? (PS, I'm quite a noob, but I have a tech group I can push ideas onto and maybe get them to make it happen). Thanks.