Re: What would you do?
Well if cost is truly a concern.
Your thinking about your shotgun, as if it were a rifle but you plan on using it as a shotgun. Just not the same animal with bird/buckshot, go shoot the chit out of it, and youll soon realize, that after a while, you will be able to point and click with little in the way of aiming...especially in close quarters use, which is what your after and youll be suprised at how accurate the bead can be should you aim.
Long story short, you dont need the aftermarket sights or the trouble it might bring to make them functional. If you really wanna save money, see if the shop will take the short barrel back, and simply cut your existing barrel down. You wont get down to 18.5 cause of the rail, but you can get under 20 and flush cut it on the front side of the last possible rail post. The remaining middle bead would be effective for sighting purposes.
With that said, good luck, your confidence in your weapon, is greater than my opinion of what you choose to do.