Upgrades to units that you shouldn't take are hornblowers and drummers. Never. Ever. Period. A waste of 15 points that can be better spent on more bodies
For Easterlings, I would recommend taking a Dragon Knight: they are cheaper than a Captain, have a higher fight value and still come with a point of might. Banner bearer's can be useful (particularly since they can help to prevent you from becoming disordered).
As to your list, it looks okay, but the formations are all to small. Haradrim are very squishy, so I would say 5 companies each is best (unless a unit is ridiculously tough, then 4 is the best formation size IMO. The lower defence, the more companies needed). The cavalary could do with at least one more company each as they are likely to attract a lot of shooting and, although they have a higher resilience, they have lower numbers of models: 4 hits is a lost company compared to 8.
Bows on cavalry are likely to be a bane more than helpful (unless they have skilled or expert riders [I don't have the book to hand], but I don't think that they do), as you'll have to move at half-rate to use them. Spend those 15 points elsewhere
As great a deal as it was, it may be worth considering dropping the Mumak to take Khamul/the Betrayer and some more companies (or just both Ringwraiths). As for allies, there are 6 ringwraiths that are a part of your army and 3 that would be allies (although any on a Fell Beast count as an ally...).