The objective is to get HOTWIRE to make the replacement for you... it may take a bit of effort, but they should have to 'add' something to the reservation in order to make you 'whole', as you purchased a 5 star hotel and their current offer is for a 4 star property; they can't make that swap without some additional form of compensation. This doesn't mean that they are going to give you the top floor suite at Caesar's Palace, but you should be able to do better than a swap for the Westgate without any additional form of compensation.
Based upon what you have told us thus far, i would look for either of the following:
1) indicate that you booked a 5* hotel and that is what you are expecting. As there are no other 5* hotels in the Summerlin zone you are willing to have them re-book you at the Trump at no additional cost to you (tell them that you will actually take any 5* hotel, but you assume the Trump is the least expensive, and you are trying to work with them given you understand the difficult situation that they are in)
2) if you're willing to change hotels another option would be to see if the Red Rock Resort will honor the reservation for the second nite (the nite they still have availability) and for the first nite you're willing to accept the Westate in exchange for $50 Hot Dollars as you would like the first nite to be free which is standard industry practice for when a customer is 'walked' (assuming you will use HOTWIRE again in the future and could use this version of their 'gift card')
3) take the Westgate replacement but ask for $100 Hot Dollars in exchange for accepting the lower star rated hotel
I would consider these 3 scenarios to see if any (or all) of them are acceptable to you and rank them in your preference order. I would then call HOTWIRE and tell them you understand that they are in a difficult situation since they can't force anyone to give you the room that you have already booked, but that you booked a 5* hotel that you're not going to accept a 4* replacement two months after your purchase and with just a month before your arrival, as you have already made other plans based upon the location of the original hotel booked. I would then tell that that you're willing to accept (insert your first choice here) and see how it goes. Be firm in your stance, but don't ever resolve to screaming, yelling, or getting irrational, as that won't get you anywhere.
Oh yeah, and as soon as you call ask to speak to a supervisor (and if you're asked what it's about, just tell them that HOTWIRE is canceling your reservation and trying to replace it with a lower rated hotel and that you would like to speak to a supervisor about it)... the initial contact personnel at the call center will not likely have the authority to handle this situation, so save yourself the trouble of having to explain it twice.
Good luck and let us know how you make out... remember that HOTWIRE wants to sell you a hotel room and having the hotel not honor the booking is not their ideal situation either; be calm and rational, and i'm sure you'll reach some sort of amicable solution.