A lot of questions so I'll just jump around because my brain is too old to be any other way. First removing the dowel or the "magazine follower" and replacing it with a shorter model (should be available aftermarket but I don't own a Maverick or any Mossberg after giving the one I had to my daughter and it was a bolt action) should be easy, safe and should not cause any problems. Some shotguns need a longer spring if you go with a shorter follower. Again I'm not that familiar with the Maverick.
Shotgun ammo (shells - not bullets) are often dirty critters. They leave residue in your shotgun which can build up pretty quickly depending on the ammo you're using. Some guns I own I hardly ever clean. I clean my shotguns every time I shoot them.
The finish on your shotgun should determine how you oil it. And almost all guns, particularly inexpensive shotguns, will benefit from oil. I have an 870 Express that I oiled probably 100 times with as much oil as the finish would hold. That's because that type of finish needs oil to really protect against rust. I haven't had to oil it in years now. And it doesn't have a speck of rust on it. Some people complained about their 870's rusting in a very short time. That's because they didn't know how to oil it and store it. NEVER store your shotgun in a foam lined container unless it is specifically made to prevent rust. That rules out any of the cheaper models. Keep it dry and lying down flat so dust doesn't settle in the barrel and action.
NEVER use birdshot for self defense. Birdshot is for birds. I've seen too many animals walk away from a blast of birdshot like they didn't even notice it including things as small as chickens. That's right. I shot several that were eating my tomatoes (my neighbor told me to). They didn't die. They didn't even flinch. I was about 15 feet away from them. And no I didn't miss. I've been shooting for over 50 years. I shot a shotgun when I was 7. My first shot I hit a clay pigeon flying.
00 buck is best for HD. If you want to look at tests look for tests that involve shooting through brick which is on your house. Sheetrock is not hard at all. It is a very poor option for stopping a bullet other than a few 5.56 rounds designed to get pulverized by contact with almost anything. Buckshot will do damage to almost anything except steel. Brick will stop most of it but not all of it. Pellets can continue on for several yards and still be lethal. Remember that a single pellet from a 00 buck shell is the same size as a .32 caliber bullet more or less. But they are round and don't travel like a bullet. There are technical terms for what shape travels how far. That's on down the road for you IMO.
Practice makes perfect. Don't store your shotgun with a round in a chamber. Shotguns are not made for that especially pump shotguns. Don't lean it against a single wall. Lean it in a corner if you must lean it at all. Laying it down is best. Wall mounts are good.
Probably the most popular round is Federal Flite Control. It doesn't spread out as fast as other rounds.
And it's an ammo tube or magazine.
Asking questions is the right thing to do. Getting help from someone who knows about guns where you can see what they're doing and why in person is the best thing. That might mean getting instruction from a class or teacher or it might mean finding a friend that knows about guns. There are safety rules everyone should know. Please learn those first thing. They are posted in many places on the net. Do a search and you'll find them. They aren't complicated or anything. Just common sense.
