That is an unfair comment in my opinion as to the very present day a Hohenzollern is head of the Royal House of Romania. That after 90 years (!) of being a Prinz von Hohenzollern Michael suddenly decided to sever the ties (2011) does not change that fact.
We can not blame Michael von Hohenzollern...
That is equally unfair, incorrect and again, a fallacy, as you are reducing a former King, whose highest-achieved title is the one he is to be referred by, His Majesty, King Michael I of the Romanians, and not your constant 'Michael von Hohenzollern', which is neither his name, nor his title, nor his title of pretense, nor his highest title that he gets to use for life, as per royal, diplomatic and curteous custom.
When Romania became a democracy, naturally the Romanian Royal family (Michael, Anne and their children) were the focus and the previous and current Fürst von Hohenzollern were wise and polite enough not to seek the limelight. What else can one expect? Prince Karl Friedrich or his son Prince Alexander walking around in Budapest and doing official things or so, while fellow Hohenzollerns reside in the Elisabeta Palace? As long as King Michael does not invite them, introduce them, they can not be blamed for being invisible, low key or "not interested".
There is no evidence anywhere that any Hohenzollern has not been invited to Romania, so that is an assumption you cannot make. Unless you have access to leaked correspondence or other evidence that any Hohenzollern wanted an invite to a function in Romania and was denied, that argument is not valid and should cease being used.
Like any German, any Hohenzollern is free to travel to Romania, just as any other citizen of the EU, and there has been no private or public visit to Romania by any of them.
The only facts we know, are the ones that have been published, 1) by Prince Karl, who stated that the succession to the Romanian throne was a matter for the Romanian Royal Family, and that his family had no interest in it and
2) that the King issued a proposed new line of succession that he encouraged parliament to consider, nothing untoward and only a proposal, that the republican government clearly is approving of, as they are now accepting the Crown Princess as heir to the dynasty and Royal House of Romania, upon her fathers illness, and with the negotiated deals made into law this summer, concerning the Royal House.
That firmly establishes who is the Royal Family of Romania onwards, and while I have no basis to claim that no Hohenzollern will again be invited to sit on the Romanian throne, it is beyond unrealistic, and while we have no evidence today of Nicholas being invited into the royal fold again, it seems unwise of the Royal Family not to, all the while he is quite literally the only future member of the Romanian Royal Family who has the stature and the interest to date of being what Romanians might want him to be, a monarch.
All that being said, we can speculate until winter has come, and gone, about what will happen to Nicholas, and that is what this particular thread is about, but to say that he should be a tool for monarchists who seek to displace his own family in a monarchic future in Romania, is both disrespectful and with all due respect, a ridiculous stance to take.