I think, given the etiquette and balance of power of the Imperium, Leto could have refused, but that would result in a loss of prestige and privilege - and, more importantly, of opportunity. The Emperor and rival houses could use this as an excuse to exclude the Atreides, and refusing such a challenge would undermine the Atreides reputation of bravery and honour. It might be more of an insult rather than something illegal, but insulting the emperor gives him an excuse to retaliate more directly. But I think "why should we listen to you at the Landsraad if you can't govern a planet the Emperor gives to you?" would be a hard question to dodge.

But at the same time, the Atreides are still as subtle as any other house, and are maneuvering to succeed - this is a major risk, but if they succeed, they would greatly diminish their Harkonnen rivals, would greatly increase their wealth, and would have been seen as beating the emperor at his own game while maintaining their reputed virtues. That might even add up to enough power and influence to directly take the the throne.

I think, given the etiquette and balance of power of the Imperium, Leto could have refused, but that would result in a loss of prestige and privilege - and, more importantly, of opportunity. The Emperor and rival houses could use this as an excuse to exclude the Atreides, and refusing such a challenge would undermine the Atreides reputation of bravery and honour. It might be more of an insult rather than something illegal, but insulting the emperor gives him an excuse to retaliate more directly. But I think "why should we listen to you at the Landsraad if you can't govern a planet the Emperor gives to you?" would be a hard question to dodge.

But at the same time, the Atreides are still as subtle as any other house, and are maneuvering to succeed - this is a major risk, but if they succeed, they would greatly diminish their Harkonnen rivals, would greatly increase their wealth, and would have been seen as beating the emperor at his own game while maintaining their reputed virtues. That might even add up to enough power and influence to directly take the the throne.

I'm just thinking of how much subtle maneuvering there is in Dune. Even if you don't trust the Emperor, saying it so blatantly would be a terrible political move. I could see it leading to a revocation of titles and holdings as he's refusing to do his duty, but it could just lead to being blocked out of anything important and essentially dropping down to a Minor House.