Is there a best of breed Makarov or pseudo-Makarov, like the CZ 82/83, aside from the near impossible to find E. German ones?
First, with the nitpicking... pseudo-Makarov isn't quite accurate, but I suppose that's beside the point. It's hard to say what's best because that may depend on what your preferences are. For example, if you want the most concealable one (the smallest/lightest) it would be the Polish-made P-64. Of course there's the drawback of having the smallest ammo capacity of all of your choices (6+1), and also has nasty recoil and a heavy double action trigger. The recoil and trigger pull can be tamed somewhat using affordable, easy-to-install aftermarket springs, but the magazine capacity is something you're stuck with. The CZ-82 has the advantage of ammo capacity (12+1), although some Russian-made commercial Makarovs also have 12-round mags. It has a decent double action trigger, but it's the bulkiest and heaviest of the 9x18's. The actual Makarovs (regardless of what version) fall somewhere in between. There's some variation in fit and finish, but all of these are going to durable, accurate, and reliable. The only 9x18 I wouldn't whole-heartedly recommend is the Hungarian Pa-63. They're not bad guns all around but the aluminum alloy frame reportedly has durability problems.
Is there one that has a first DA trigger pull that is somewhat acceptable?
The CZ-82 and the actual Makarovs (Russian, E. German, Bulgarian, etc) all have an alright DA trigger pull, it's the P-64 and Hungarian Pa-63 that will give you trouble.
Are replacement parts hard/easy to find, as well as magazines?
Somewhat. The mags for any of them aren't as cheap as they should be. The nice thing about the actual Makarov pistols is they're composed of extremely few parts and there's really nothing in them that's particularly prone to failing.
Is there a custom form holster, designed for IWB/OWB carry, that's more modern than the milsurp one?
Yes, for example there's Fobus holsters for the Makarov pistol.
Finally, why would a 9x18mm cartridge be significantly less effective than a slightly longer 9x19mm one?
It's a question of chamber pressure. The Makarov was originally designed for extreme simplicity and reliability. Made it cheaper and easier to produce, and also to easier to maintain by those to whom the gun was issued. In trying to achieve that simplicity and reliability, they figured it would be best to make the gun blowback operated, and the 9x18 was considered the most powerful round a blowback operated firearm could handle while making the firearm of a size and weight that could easily be carried.
Hi-Point has shown that you can make even a 9x19, .40S&W, or .45acp blowback operated, but basically these guns look, feel, and weigh more like power drills than pistols.
