The mumaks at least can be rationalised thusly: it would have been very difficult to take them over the Morgul Pass (twice, once each way); and there was no room for them in Minas Morgul. So to Udun they went.
Of course this would mean backtracking them to the Osgiliath crossing- but that was the only way across (after the Anduin was bridged): in fact all the heavy stuff, siege towers and artillery and Grond, would have to have gone that way. Certainly not by boat at Andros! Since the Mumakil are not reported at the forcing of the River, in fact not until the Pelennor itself, there was at least time (five days) to get them there.
This raises the possibility that the Morgul-host was the assault force at Osgiliath and the causeway forts, but that it was reinforced once the bridgehead was secured by troops from the Morannon, not all of whom crossed at Andros (indeed, simply blocking the Rohirrim wouldn't require an especially large army, and certainly Elfhelm and a fairly small force was able to defeat it afterwards.
It may be relevant that Sauron was precipitated into launching the attack early (by Aragorn with the Palantir) and his logistical planning was thrown into some disarray. The collision of Orc-columns later near the Isenmouthe, which allowed Frodo and Sam to escape, must be only a very small illustration of the sort of chaos Sauron's quartermaster corps was dealing with on March 9-10!
To get all fanficcy, one might suppose that under the earlier plan before it was disrupted, the Anorien-force included engineers whose task was to throw a bridge over at Andros (Caesar bridged the Rhine in ten days), and that the heavy Morannon forces would have proceeded thence and moved on MT from the north- this would at least simplify the supply difficulties inherent in relying on a single bridghead.
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They could have been Men of Mordor - maybe some were Black Numenoreans, or half-bloods, maybe others were Haradrim in origin but living and serving in Mordor,
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Why not just Haradrim? No Black Numenoreans or Men of Mordor are ever mentioned in the battle- the only cavalry was Southron.