It didn't work, sadly. I can move while attacking and then the character gets stuck in his first walking frame whenever he's standing still. Don't worry though, my temporal fix is still working fine so I can continue testing everything else.
What did work, however, was the rapid fire fix. I had to put "Trigger once while true" on event 25 though, since putting it in 23 made the "spawn another object" event useless.
By the way, the bullet only goes to the right at the moment. What do you recommend in order to make it go up, left and down, accordingly? Right now the bullet is set to spawn at the third (#2) animation frame. I suppose I should try to use "set angle" to change its trajectory, or should I create a new object with a mirrored/upside-down bullet?
I apologise for the tardiness in my reply. I wasn't home, but please keep in mind I'm very interested in getting this to work and I appreciate your help.