Why do you think any kind of spark is allowed by Zarzuela ? JC and his camp didn't even like his own son's (Felipe's) spark when he was ill, let alone the 'so called' spark from his DIL. Plus each country is different. If Letizia were given more prominent role as Maxima in Netherland, outshining her husband, sometimes walking in front of everyone, maybe it even made the monarchy less secure
. I don't see how other commoner born royals would have been criticized less in Spain if they were on Letizia's position since lots of criticism were out of classists and sexists (ex. Jaime Penafiel) or their own political agenda (Republicanism).
Zarzuela pretty much allowed Jaime Penafiel criticizing Letizia freely with invalid datas (mostly rumors or lies) at the beginning (I think that was their mistake), now it has become a good business talking about her since anything (good or bad) related to her still sells. I don't know any other royal (maybe except for Kate) would have their bikini pictures sold for 300,000 Euros.
She is only wife of the heir, not on a position to bring the monarchy forward. As long as JC is on the throne, nobody (not even Felipe) can (or is allowed to) bring the monarchy forward other than JC himself.