Do you agree with "The way to make room for more participation is to take down any barrier for participation"?
In general I would agree with this, yes - but, as my opponent said, constructing new and novel ways for people to participate is also vital.
What conflicts and limits does Sovereigntism has towards our foreign affairs and traditional treaties?
The fact that Sovereigntism is related to, but not identical to, defenderism is probably its biggest weakness. Sometimes we will conduct operations like those in Madrigal - invading raider regions - and we will meet, at best, apathy from other defender organizations and outright opposition at worst. I think it's important for Sovereigntists to maintain their independence from the defender movement at large and not allow occasional disagreements such as this to impact their pre-existing relations and treaties. There is no point in dissolving our treaty with the Rejected Realms, for example, because their army opposes a raid against a raider region.