Finally, I get back to you and your question, and not before time!
‘What’s the historical significance of Simon de Montfort and his parliament(s)?’
Short answer: much less than is usually attributed to them by those looking back at them through the lens of 600 years of history.
It would be anachronistic to describe Simon de Montfort as a classical liberal. It is important to remember that his activities took place in the context of the more of less permanent struggle between the king and his semi-independent barons.
Some have said that de Montfort’s first parliament stripped the king of unlimited authority but that can’t be right for kings in those days didn’t have unlimited authority! Kings were simply primus inter pares, first among equals.
In the matter of summoning ‘ordinary’ townspeople to parliament, this is not as revolutionary as it may sound when we realise that parliament was primarily a tax-granting body, not a law-making body and its purpose was to provide finance to the king.
‘Who is the father of British constitutional government?’
Pick a card, any card! I’m disinclined to think of any one person as playing that role and would be more inclined to trace constitutional government to the emergence of the supremacy of parliament after the so-called ‘Glorious Revolution’ and its gradual transition from a tax-granting to a law-making body and, especially, the emergence of the cabinet system and prime ministership, when the German-speaking king stopped attending meetings of his ministers.
I hope this helps.