My dear friend, you underestimate Sauron's army and overestimate Voldy's power. First, the searing heat from Sauron is ancient and due to his malice (Isildur said it kept the Ring's inscription visible, and we know it takes a flame to do that). Second, Sauron knew better than Polyjuice potion, and anyway, Voldy would not take the heat produced by Sauron. Also tell me how would he get past the Nazgul and Mordor's desert. Third, orcs know only to destroy their enemy, and from what I know, it is not possible for 1000 people to control 1 million orcs - even the human mind has limitations, and controlling even itself is not an easy task for it - how is the mind controlling 100 people at a time? Also, Cirith Ungol was due to a personal feud between the two orc captains, and in a war, orcs RARELY do that. Moreover they only follow what their master tells them to do. Fourth, Sauron is one of the most powerful Maia (did you read Unfinished Tales?), probably only behind Eonwe. Fifth, Sauron and the Nine are extremely powerful sorcerers, and they would SURELY know better magic than the normal death eater, and therefore the arrival of the Nine in the battle would make the Protego charm useless, as the Nazgul are undead, and should be able to stop anyone from conjuring magic in their presence (they are known to cause a lot of dread upon even hearing about them), and destroy magic. Unfortunately, Tolkein refused to give a detailed list of the abilities of the Nazgul and Sauron. Sixth, while not mentioned, apparition would be easily hampered by weather if stromy conditions occur over a large area, and anyway, the Nine's dread makes it unable for anyone to do anything other than fleeing to a safe spot - magic is impossible if the Nine appear anyway as they are sorcerers who can disable the DEs from apparating long distances or even apparate by some dark magic. Again, few abilities of the Nazgul are known. Also, your argument that Voldy's protego would stop the Nazgul is useless if the Nazgul can break the charms cast by the DEs, as it is very likely that the Witch-king has more power than Voldemort (cannot be confirmed due to lack of information on Witch-king, again).
Lastly, every orc Sauron loses, it is not a problem for him - he breed orcs in Barad-dur continuously. Not same for Voldy as every life lost is a setback to him. Also, on a better look on the potion, Mordor is pretty enchanted, making it difficult for anyone to use magic there (save for Sauron and the Nine) and one does not simply walk in to Mordor, or reach Barad-dur form the Back Gate in 1 hour.....Although I shall do a bit more research to back up what I said.
[doublepost=1464175603,1464175459][/doublepost]Yes, Jerion, Maiar are clearly stated to be on the level of Demigods in the Silmarillion (more precisely its second part the Valaquenta) and are known to be extremely powerful sorcerers, and Sauron is explicitly stated to be one of the most powerful Maiar