De Klerk did support a return to monarchy. In 1993 he told Mandela that a return to monarchy would be a return to normalcy and would allow the nation to get past a horrid three decades. The rest of the NP is obviously republican leaning but considering their leader was in favour and that there was a free cross party vote, it does mean that the NP as a whole would not oppose it. Their power base had been reduced radically since 1991 and De Klerk was eager to get it back to shape. If the ANC supports the monarchy then the NP will.begrungdingly do it too.
I am very skeptical about any allegations that F. W. de Klerk supported the restoration of the Monarchy in South Africa, and would most certainly have to see a few credible sources back that up. The National Party was the vehicle and backbone of the Afrikaner identity and movement in South Africa, and thus represented all its aspirations and ideals: the primary of those having been an intense dislike for the United Kingdom and all its institutions. The referendum and declaration of the Republic in 1961 was the long-term realization of such aspirations, and even if there is some remote idea that de Klerk did in fact support a restoration, I highly doubt he would've been able to cajole the rest of the National Party into supporting that - he had already spent much of his political capital in bringing about negotiations and the end of Apartheid.
Furthermore, to that end, even if Mandela for example was amenable to such a notion, the entire point of the negotiations and the transition was the building of a stable and agreeable post-Apartheid future, one that could placate even the radicals within the liberation movement (especially those within MK and the SACP). The idea of bringing back what is arguably viewed as a "outdated colonial vestige" would deeply upset them, and also anger large swathes within the NP (as mentioned), and not to mention seem unpalatable to large portions of the South African population. In effect, all it really is going to do is cause more instability and more issues, angering an already tense population in a tense era. I mean, altogether 1989-1994 was a very chaotic period, one filled with a lot of violence.
And yeah, another issue on top is if you're restoring Queen Elizabeth II, what of the rights and powers of the various local monarchies? The AmaXhosa, the Zulu, etc.; what would their structure and functioning then be under this system? It's in effect really a can of worms, and I cannot stress that idea enough.
Altogether I am basing my own assertations here on my experiences (including what I've learned within academia in SA), and from friends/family/etc., especially those around during the 1980's and 1990's.