The biggest issue your system faces is a quasi-guaranteed stalemate for almost any topic.
On the other hand, any democracy that does not protect their minorities is not a democracy at all, but an Ochlocracy (the tyranny of the majority), so your system, while inoperable, would at least not fail in that aspect.
Your system would work if benevolence of all members of parliament could be guaranteed. This, of course, is impossible with humans.
But if you look at contries with more than two parties, like for example Germany, where there's typically four to five parties, you normally have coalitions formed from the party that got the most votes and whichever other party can help form a majority and is wiling to cooperate.
To protect minorities, there is a court of law that makes sure all laws passed respect the constitution (well, in theory, but that is a somewhat different story), plus there is a second chamber with different majorities, to balance the first.
The coalitions already help with balancing things, not least because after the next election, you may need to form a new coalition, so it's best not to offend the other parties too much. After all, such a system always mean that you can have a valid coalition without the strongest party, as long as they are below 50%, which would be normal in most multi-party systems.
What does that mean for your question:
Unless you find a way to solve the benevolence problem, you should not allow a single opinion to block legislature.
Your supreme Mugwup has the same problem. If he was guaranteed to be benevolent, you would not even need parliament. And if he's not, the entire veto thing is already down the drain.
You should want a political system that allows for more parties. That will probably mean representative seats for the different parties, and no winner-takes-it-all systems. It will also mean you need a careful set of checks and balances.
All in all, a "good" political system is one that prevents an individual group or institution from amassing too much power. And it will need constant fine-tuning, while still allowing for actual decisions to be made in a reasonable time frame.