You are correct in that when using a clear over the base, the base is always a matt finish. I suspect it is because that type finish of allows the metallic flakes in the paint to lie evenly to give you a nice finish. If you have ever tried or seen a metallic painted as a top-coat gloss you'll understand why because if you aren't careful the paint dries at different rates and you can see color variations in the paint. Look next time at a car show and you'll pick the differences.
The matt base is always meant to have a clear over the top. However, the paint is a matt because it has been designed like that and they wouldn't do it by specifically adding a flattening base. The difference is that when you want to apply a clear coat (which is normally glossy) you add the flattening base to give it that satin look.
That is the clear over base (COB) system. You can also paint it as a single coat. In that case, you would take a normal top-coat paint (which would be glossy) and add the flattening base to make it satin.
The question of hardeners is different and it comes down to personal paint choice and what is available. The 2K paint systems (with a hardener) will give you a tougher paint and the 1K (non hardener) paint systems are less durable but makes painting simpler. The base can come in either 2K or 1K versions as does the clear coats. The rule of thumb is that if you choose a 2K base then you have to use a 2K clear coat and that is the same as the 1K systems. You aren't meant to mix them up. If you put a 2K clear which is hard, over a 1K base (which is softer) you can get cracking as the clear can't move and flex with the base coat. Though it's not recommended, you can cheat and put a 1K clear over a 2K base as the clear is the softer of the two. The down-side is that the 1K clear is not as durable as a 2K clear but that is not much of a problem of a low-use Mustang.
I hope this helps a bit.