I tried to test it last night,
nice to see Keyboard is now being supported.
The overall controll getting scheme is pretty nice

I wanted to add an AI Player but it wasn´t possible to click on the Add AI Button with the mouse.
Using the joypad I played some games against the stupid AI.
I tested many characters but couldn´t find the invincible bug you have mentioned.
I only found two odd things so far:
The contra stage is pretty strange, if you only have characters that can´t jump high enough to get out of the water holes on to the land.
On the Mario stage I got stuck with goomba in the corner between two blocks ( screenshot attached )
There are some nice maps designs included in this one.
Some characters really had much thought behind them, I liked the snowball character the most so far.
Some enemys have the option to change to different modes, I noticed speed differences of the characters when I changed modes,
when I was with goomba in the blue mode,
the special bar only got drained when I was jumping or falling, even though the character can also run faster when he is blue, is this intentional?
Characters like koopa, the shielded enemy from mega man ( if I manage to do a normal shot with him again ), where great,
also all the other characters that seemed finished.
The Lakitu has an attack, which drops a ?, that was the only time the AI got me

does it always do 300 Damage, or is it a random amount, could this also heal by a small chance?
Hope to see some more updates of this.