Reagan's justification for invading Grenada was that he was protecting the hundreds of American medical students studying in Grenada.

The Crimea was part of Russia until 1954. It had, and still has, a massive Russian military base there. In 1992 Ukraine officially recognized the Crimea as largely autonomous. So in that light, Russia has much more standing to go into the Crimea then Reagan had to go into Grenada.

This is completely irrelevant - a big mistake does not justify a different - and completely unrelated - "smaller" mistake in any way.

It's not irrelevant, it just shows how governments "think". This is more or less accepted practice in foreign affairs.

I did not want to imply that the comment is irrelevant, it shows, as you say, an interesting parallel in the way of thinking. I wanted to say that it is completely irrelevant as a justification for the current actions of Russia.

Not to mention that the population of Crimea is over 50% ethnic Russians.

That's how the "Five Eyes" operate, so yeah.

The Five Eyes rationalize that it's perfectly normal to spy on enemies together, and share information, given that the Five Eyes nations all speak English.

https://en.wikipedia.org/wiki/Five_Eyes

Well it's a little deeper than that isn't it? There is a lot more common ground than just the language, like 4 of those countries are CommonWealth realms with the Queen as the head of state, so there's a big distinction right there. They also have similar military structures or processes since WW2, a history of major collaborations (like the Manhattan project between Canada, UK and US).