A few thoughts...
I like how it all ties neatly together: The same bonus used for bonuses to actions/EDCL, EDCL ending at 20, etc.
There are effectively only 7 character levels, this means "level" will increase less often than is typical in D&D. Gaining levels and small perks is fun! Might be an advantage to make it more granular, maybe not. I realize that EXP = level, more or less, but still you need to gain quite a few levels to move up.
I assume that two legendary characters aiding an action does not equal +24 to that action. An idea might be that highest bonus counts, +1 if there is an equal level character helping (if the bonus was already above 0) and no further advantage to assistants on the same action. Or something similar.
Not completely clear on what happens if an assistant is helping a regent action - providing able assistance - that is. What action is used for this? The Ply Trade action?
Having a separate proficiency for Contest Law Holding and Rule Law Holding might be a bit over the top, imo. One proficiency for each Holding type might be enough. Already a lot of ground to cover here.
I like how ECL and EDCL makes priest, druid and mage domains less dependent on their regent. There is still a big imbalance here - in a realm domain (law, manor and provinces) it hardly matters what level your regent is. It still matters a lot for caster domains... Providing some kind of perk for having a legendary fighter ruling you realm might be cool.
How does this tie into adventures? Exp level and character class is used?
That is a lot of different classes. A lot of them are just variations on the same, almost identical, theme.
One way to solve the different Regency Collection rates might be to say that a starting character collects full province regency + two full other types (or 1 full, 2 ½; 4 ½). Then adds ½ a holding type at X level and ½ a holding type at legendary. So a legendary Wizard might collect full Province, full Source, full Law and full Manor... a Legendary Fighter might collect full Province, full Law, full Manor, ½ Guild, ½ Trade.
Possibly spread it out a bit more, start with lower collection, etc. That way you are neither rewarded nor punished for multiclassing, with regards to Regency collection anyway.
"Bound" bonuses is a good thing. Keeping the bonus between 0 and +12 makes thing manageable and the math easier.