Toby,
The muzzle-loader will be the easiest scenario, because you are loading the bullets in a sabot and it doesn't matter if they are cast or jacketed. Make sense?
Well, there is a LOT of information about cast bullets, so let's stick to the basics: They need to be the proper shape for making a good wound channel (flat nose, example the WFN, LFN or other Veral Smith designs or the "Keith" semi-wadcutter of Elmer Keith fame), and hard enough to withstand pressures/velocities - except in the case where you shoot them out of a ML with a sabot.
At the same time, if they are too hard they can shatter, but at the velocities you are quoting, and on typical Missouri whitetails, nearly any decent cast bullet should be fine. Even the cheap stuff from Midway ought to hold up - I've shot them through tree limbs.
A gas-check helps prevent leading when pushed to maximum velocities/pressures. It has no effect on terminal (wound) ballistics.
OK - let's talk about your jacketed selections. First, you did the right thing by going to the manufacturers, they can help. Likely, your ML is just a bit faster than the XTP can hold together - after all, you don't get these velocities with a .44 mag revolver loading.
The 265gr. Hornady was brought out for the .444 Marlin rifles, they had the same problem you do, bullets for typical .44 mag velocities don't hold up from fast rifles.
The Gold Dots are plated, not a typical bullet construction, and have a reputation to hold up well, as does the 300gr. PSP Speer that you mention.
The '.454-rated' bullets will be quite tough, and might be just what you need for the speeds your ML will run. But - at long distances they might not open up too much. Probably something that bears experimenting with, try wet newsprint for an expansion medium, maybe some bones thrown in to make the test more severe.
If you stay toward the heavier bullets in any caliber, you should generally be getting less expansion, because you normally can't run them as fast.
Hope this isn't too much information all at once, fire away with any questions.....