Monarchists are 10x more nationalistic and patriotic than the IRI and its Islamist supporters. If nationalists are in charge not only would Iran have the support of the US and West (which in itself would deter any aggressive moves against Iran) but they would also not allow any separatism. The big voices in the West would also cease lending their support to separatism inside Iran. The West doesn't care about Kurds or Arabs or Baloch populations, they just use those populations to undermine Iran because Iran is currently their enemy.
I'd have to differ categorically on both counts.
Monarchists are led by and are obedient to their crown prince Reza Pahlavi. A study of that character's and his close associates' political shenanigans over the past decades will be telling in this regard. Pahlavi and his clique have fully submitted to the balkanization agenda for Iran as defined by their zionist and western patrons. Hence the recurrent meetings between Pahlavi and separatist elements (including Azari ones), hence his recent statement that separation of provinces can be put to vote since "democracy" trumps everything else, and so on and so forth. This is amply documented, so I'll restrict myself to mentioning the existence of this vast body of evidence, which is accessible through online search.
Moreover monarchists at large have undergone ideological evolution as well. They're now heavily influenced by globalism and liberalism, therefore their nationalist background has been watered down considerably. Not to mention that modern secular Iranian nationalism had been affected by western (read Anglo-Saxon and French) cultural imperialism from the outset. The memory of a figure such as Taqizade, who used to openly advocate "westernization" of Iran as the "sole possible" path to "modernization", and to consider Iranian culture as "inferior" to the west's, is still fresh. Now yes, westernized elements of this kind exist in the Islamic camp as well (reformist and moderate liberals), but who will be calling the shots among Islamic forces in the event of a major security crisis, and who will be doing so within the monarchist one? The answer should be clear I believe.
To compound the point, so-called "federalization" of Iran's state institutions along "ethno"-linguistic lines, complete with the establishment of regional governments and the providing of all public education classes in local languages - a textbook recipe for balkanization, features prominently among the declared political programs of every single anti-IR group that matters at the scale of the opposition. And this includes the monarchists. It's part and parcel of their goals, it's official. How this became an irrevocable item on the agenda of the monarchist opposition is self-explanatory: it's that their western and zionist sponsors have requested it from them. Ardeshir Zahedi, a former minister active under the shah regime denounced this, but what happened next? He instantly came under sustained fire from the monarchist establishment and turned into a dissident for the rest of his days.
This "ethnic" federalization program, an obvious and definitive prelude to territorial disintegration is therefore the absolute best case scenario awaiting Iran if the current political order were to make way for another. Truth is that the Islamic Republic represents indeed the ultimate line of defence standing against the concrete implementation of this sinister watchword.
When it comes to western regimes, they very much care about making sure that a potentially independent Iran won't challenge their interests ever again. The most foolproof way of achieving this, as per the Oded Yinon doctrine, is by subjecting the target to territorial dissolution along "ethno"-linguistic lines.
USA and NATO policy in West Asia is largely determined by the zionist lobby in Washington. And the latter will not let any opportunity to break Iran into pieces go to waste. But even assuming that western regimes have been autonomous in their policy-making toward the region: a cursory glance at the fate of nation-states confronted with western-orchestrated "regime change" operations and other violent interventions, from Sudan to Libya via Iraq and Syria will remove any doubt as to their game-plan. And, Iran to them is the big prize. She would be given that treatment severalfold.
Equally of import is that ultimately, the will to preserve territorial integrity is not a function of how nationalistic a leadership is. For this is something any functional government regardless of its ideology will prioritize. Historic examples of states following an ideology other than nationalism but going to great lengths to safeguard their territorial integrity aren't hard to come by.
As a matter of fact, nationalism hardly makes sense and will be reduced to a facade devoid of substance in the absence of independence and self-determination - precisely what the monarchy was sorely lacking, and which the Islamic Republic restored in Iran. Neither Reza Khan nor Mohammad Reza Pahlavi could muster what it takes to uphold Iranian sovereignty in the face of imperialist powers; Reza Pahlavi junior being the most farcical of the three in this regard.
Of course that is assuming a "clean" regime change which is unlikely. IRI supporters and officials are not likely to cede power peacefully like the Shah did, they will probably burn the country in a civil war instead. But that's effective, that's why even a lot of people who don't support the IRI don't want another revolution.
The shah unlike the Islamic Republic was dependent on his imperialist bosses. This was so when he fled the country in 1953, and it was so in 1979 as well. Except that in the latter case, western regimes too were powerless in the face of what was one of the most authentic, momentous and earthshaking revolutions in history.