It's not uncommon for the gooseneck pin to slide in and out a few inches. As long as it doesn't come completely out, it's probably ok. The mainsheet tension will keep the boom pressed forward and prevent it from sliding back while sailing. However, if the sliding pin annoys you, then you will have to pull off the end cap, remove the clip on the end of the pin, slide a piece of rubber tubing over the pin and then re-install the clip.
You should not need to remove the end caps to replace the outhaul cleat. All you need to do is remove the screws in the cleat and install a new cleat. The only exception to this would be if the screws are seized in the mounting bracket then you might have to replace the bracket, screws, and cleat. Be sure to replace the cleat with an aluminum one, not plastic. The plastic cleats slip, the aluminum ones don't.
In order to replace the outhaul line, you'll need to pull off the rear end cap. In that case, you should replace the return bungee at the same time.
The best method to remove pop rivets is to drill off the head only. I usually use about a 1/2" drill to do this (same diameter as the head of the rivet). Then use a punch and hammer to drive the mandrel into the boom. This method assures that you won't oversize the hole in the boom.
You should use stainless steel rivets at the gooseneck end of the boom. You can use aluminum rivets at the back end of the boom. They are 3/16" diameter. Home Depot sells aluminum rivets, but not stainless. The best place I've found to get a lot of stainless rivets (more than 20) is McMaster Carr online. If you only need a couple, you can go to your Hobie Dealer, Murrays, or West Marine.
Keep in mind that it takes a LOT of hand strength to pull stainless rivets using a basic pop rivet gun from Home Depot. Also, be sure to wear safety glasses when pulling SS rivets.