Both of the questions you are asking are very reasonable things to want to accomplish. While you may never get the chance to shoot more than one arrow at a game animal, it is not unusual in a tournament to be required to accurately fire multiple arrows at a target from the same distance, which is as much an archery skill as learning to shoot arrows accurately from different distances.
My guess is that your 18-20 yard problem is just a mental block. You use exactly the same form for shooting a 10 yard shot as you do for a 50 yard shot. This is why blank bale work is so effective: you can improve your 50 yard shots by shooting at a blank bale 5 yards away! Your groups will open up some from 18 to 20 yards, just because a group of arrows disperses with distance. For example, two arrows that are 2" apart at 18 yards will be something like 2.3" apart at 20 yards, due to natural dispersion with distance. Any more of a difference than that means that you're shooting differently at 20 yards. Try playing a trick on yourself: pretend that the 20 yard shot is an 18 yard shot. Aim, or shoot instinctively, as if the target is 18 yards away. You should be able to shoot arrows that are nicely grouped a few inches below the bullseye. Once you can do that, maybe you can convince yourself that there is really no difference shooting at 18 or 20 yards, and move the 20 yard group up where it belongs.
To shoot groups of more than 2 arrows accurately, you need to develop a similar mindset to the one you would have when you know you're only going to shoot one arrow from a given place. You need to apply this mindset to each arrow you shoot. In other words, you are not shooting a sequence of arrows, like bullets in a belt of machinegun shells. Each and every arrow you shoot has a distinct set of requirements to set up, draw and shoot, that is not related to the last arrow you shot or the next one you plan to shoot. This mindset requires discipline and must be learned, just like any other archery skill. It is not an easy mindset to develop, and is one of the things that separates an Olympic archer from someone who can occasionally hit the target at 100 yards.