For the first question it's pretty simple, just add a condition to check for when the score is 5 then add an action to set the bullet speed to 500.
For adding 100 to the speed every 5 seconds you could make an every tick event with an action to set the speed with the expression:
int(score/5)*100+initial_speed
Where initial_speed is whatever the bullet speed started at, or rather what speed you want it to start at.