Bloomberg View interviews the American-born president of Estonia, Toomas Hendrik Ilves. He believes his small Baltic country has taken the steps necessary to keep Putin from doing to it what he has done to Ukraine. Time will tell if he is correct.
Yes, Estonia is a member of NATO, unlike Ukraine. NATO members are treaty-bound to come to each other's defense. Ilves is perhaps more convinced NATO would go to war to defend his country than, strictly speaking, he should be.
The more compelling reason ethnic Russians in Estonia won't decide to cast their lot with Putin is the standard of living is substantially higher in Estonia than in adjacent Russia. This is unlike the situation in Ukraine where people are as poor as Russians. A higher standard of living was the reason today's Russians, or their parents, retired to Estonia. It was a less poor part of the former Soviet Union.
Bottom line, Putin can certainly invade Estonia but he will have to do it with the Russian army. Few Estonian Russians are likely to be active collaborators in that effort.