I found that power to be practically useless. at least in the levels I got it in. because you still have to kill the enemies and you can reach them without the need to be invincible for what seems like a really short time.
There are a few places later on where you might find the log dummy to be more useful to reach a safer spot, but indeed you still have to kill the enemies in the end.
When I played I used it as a last-second lifesaver, which is the main idea I had in mind when I implemented it.
I'm not thinking of taking Mine Fu anywhere else, but you never know. I might make a sequel or something, that would most likely be called Super Mine Fu :P But for the moment the game will not change.
Regarding Lives and Scores, these are done using Variables. Variables numerically keep track of anything you want. I simply showed images of the numbers that correspond to the values for Lives and Score.
The item icon is coded just the same way: the default sword icon corresponds to value 0, the shuriken icon to value 1, etc.