If I flank a unit, hit them on their back row and then destroy a tray... leaving my unit now in corner to corner contact with their unit... are we still engaged?

34 in the rulebook under engagement: "If any part of a units trays are touching any part of an enemy units trays those units are engaged."

So why wouldn't corner to corner contact be still engaged? What am I missing here?

Per the diagram at the back of the Rules Reference (page 22?), no you aren't. You would get to close in.

When you are engaged, you don't lock trays. The trays are separated by the width of the connectors. When the contacted enemy tray is removed, your corner cannot be touching another corner of that unit.

As for the spirit of the rules, closing in is supposed to handle this sort of situation. However, there is obviously an advantage to NOT close in if you attacked first, so the opponent cannot retaliate with a simple attack.

58 minutes ago, Glucose98 said:

As for the spirit of the rules, closing in is supposed to handle this sort of situation. However, there is obviously an advantage to NOT close in if you attacked first, so the opponent cannot retaliate with a simple attack.

unless your opponent dialed in a charge in which case it is to your advantage to close it.

I have a question around this, lest imagine 2 2x1 units in combat only in contact in one of the trays. A third unit destroys one of the contact trays with a ranged attack. Does any of the units have the opportunity to close in?

If a tray is removed, any enemy unit that was engaged and is no longer engaged has an opportunity to close in, regardless of how the tray was removed.