The HRE is quite feasible, atleast for the duration of the NES. It may have not lasted throughout the centuries, but it certaintly was capable of lasting a few decades. You will remember, Silver, that nationalism, as a concept, had not yet gained significance throughout the world. Particularism, the favor of ones particular region over the rest, was still prevalent, and what is easier to manipulate than a groups hatred over their neighbors?
Case in point: The Catalans and the Germans. The Catalans, though part of Spain, hated the Spanish, and eagerly rushed to France as their savoir. Indeed, it could be said that in NES2 VI, the Catalans were more loyal than some Frenchmen. They were not swayed by Spanish revolts, for they hated the Spaniards, and France provided them all they needed. And the Catalans didn't have that nasty side of radicalism that had to be shot out of some Frenchmen
The Germans, meanwhile, were never unified except under two states, and the popularity of these two states were hardly significant. Under French rule, Germans were provided with excellent healthcare, infrastructure, and services, whilst during their few year tenure as part of the GGR, they were subject to mass conscription to fight a war which they did not really believe in. (Note the only reason the GGR came into existence was due to my essentially giving up of the German provinces I owned after the conquest of Brunswick and Augsburg so that I could focus on Spain. Stalin, however, stupidly betrayed me, allowing me to annex Germany once and for all.) The Germans, for the most part, were indifferent. This was due to the fact that France provided such services for them, and that nationalism, as a force, did not exist. The Dutch had been under French rule since before the NES began, and were profiting heavily from it. There was no reason for their resistance towards French authority. The single thing that most NESers refuse to understand is that
people will not revolt without a reason. You can not base resistance upon such ideals as national identity or social struggle if there are not realistic concerns to which you can apply your ideals. Calling for nationalistic revolts in an age without nationalism is tanamount to calling for the abolition of the free-market on the floors of the New York Stock Exchange.
The largest threat to the HRE were radical insurgents, yet without a thread of commonality, they in the end failed. They were not backed by nationalists, nor were they backed with any real cause. In the end, the Empire provided what they screamed for, and they were merely turned into a bunch of raving idealogues. The only real nationalists were the Spanish elite, yet without real planning on their part, and without the completely enthusiastic support of the people, it was only a matter of time before they were crushed.
Dar-al-Islam, in contrast, had deep rooted tensions going back hundreds of years. Europe had those too, but Europeans on the whole are generally much more flexible concerning their allegiances. Muslims and Hindus, on the other hand, are violently sectarian, as can be noted by the Sunni-Shi'ite conflict, the Kurdish-Turkic conflicts, and the Muslim-Hindu conflicts. And this has nothing to do with nationalism at all. Furthermore, you cannot found an Empire simply on conquest - after such conquest, there must be a deep rooted, and thorough attempt to establish your control over these conquered regions, and it cannot be done simply through repression. GEAR, EUA, and HRE all succeeded in part due to the massive efforts made towards consolidating their gains. Dar-al-Islam never made as strong an effort, and when it did, it simply murdered people and centralized power, furthering resentment and in the end only catalyzing the inevitable collapse.