'Last message open' is a standard attribute in Mobile Push Data > Mobile Push Demographics.
Let's say if a customer has opened your push message then using Marketing cloud SDK it communicates to SFMC and says when the message has been opened in almost real time.
So, you can use the 'Last Message Open' from Mobile Push Demographics and check with ...
Capacitor is not a platform we officially support, but I do know that customers are using it and we have not heard about any large blocking issues. Our upkeep of Cordova is minimal at this point but we do not have any plans to adopt Capacitor at this time.