The Austrian military didn’t just adopt LibreOffice; they actually contributed back to it. Over five person-years of development work went into adding features they needed. Those improvements are now available to everyone using LibreOffice, which is pretty cool.
Nah, they can give out directives and regulations to force the hand of local legislation. It happens all the time for many things. I personally have to deal with the fallout of one atm.
And I already said it costs money. And that is okay. National security costs money The EU spends several hundred billions every year on its military. A safe OS is much more important than a couple more jets. Even if it costs 5 billion euros, screw it even if it is 5 billion every year, it is still cheap compared to what it brings.