The first candidate passing 50% of the vote is a laudable theory, but that very rarely happens in the UK.
Which, of itself, presupposes that something's not working properly, surely?
Therefore AV, and all systems like it will throw up coalition after coalition governments.
No; AV will ensure that those elected represent more of the population than the current system. That actually strengthens the hands of those elected, ensures a much more equitable representation and forces politicians of all parties to compromise, thus preventing the sort of extremism which can easily damage the country. And let's not forget countries such as Denmark, who haven't had a majority government in living memory, yet enjoy a higher standard of living than we do. And they're not the only ones.
Therefore all manifestos will be worthless, as all parties will enter into a series of compromises, leaving the electorate disatisfied.
But as your subsequent comment implies, they already are...
What would be wrong with making all Manifestos LEGALLY binding after election? That might focus a few of these parties in future.
Because the only way to do that is through...Parliament, who could easily revoke it once elected.