my grandparents visit every few months; where can I get senior airline discounts?

I don't know the airline code but maybe you mean the ID of Tallinn airport ( which is EETN ) .

If an employee works for Delta (skyteam) and they wanted to fly to a place that delta doesn't, they could use and airline such as KLM (skyteam) with the same privileges Delta would have given them.

Virgin

Yes. Case in point - $11 billion from airline deregulation. Niskanen, 1989, page 659