* copied the reply in the romance group to here - and I finally understand the issue (I think) *
Rebecca, how's your gaelic?
Ri benn - king of a single tribe (lesser king)
Ri buiden - king of many tribes (middle king)
Ri ruirech - king of kings (great king)
Personally, I like rhi, brenin etc., but I like great/middle/lesser king more as it keeps new and strange words to the minimum and to be honest, it's still a story that's being told, not a history lesson. AND you'd anyways have to explain that Brenin is a great king and Rhi a middle king etc. whereas great-middle-lower is pretty much self explanatory. AND you'd stick to English!
It's not the king, lord or chieftain that is the problem, but that all their wives are referred to as queens and you don't normally expect to refer to the wife of a chieftain or a lord as a queen. I've looked, but I can't find any terms that differentiated the wives. I'm not surprised, they'd be differentiated by association to their husbands and not on their own. To be honest, their wives are not mentioned at all. Welcome to the 8th century! LOL
Unless you or someone else can come up with a brilliant fix like come up with plausible equivalent titles for their wives as not to create confusion, I don't see how you can work your way around it but to go the Great King, Middle King or Lesser King route to avoid the confusion why all their wives are queens.
To work around the cumbersomeness of always referring to them as great, middle or lesser, as I've mentioned before, you could only refer to the additional tags when you introduce them in each scene and then call them simply King from then on - their social status could also be worked in/the use of tags avoided by showing their actions in relation to other kings when there are more than one king present and in dialogue. Not all, but some readers will also quickly be able to associate each king with his "level" with or without the tags.
Personally, if this were me, I'd go the great-middle-lower route even though it's going to be massively painful (then again, changing to Brenin, Rhi and something else is not an easy fix either). It's bloody unique - as much as I understand and don't mind king, lord and chieftain, when you mentioned great-middle-lower kings, I was impressed - I have never seen it before and I had no idea until now that lords and chieftains were also kings albeit lesser kings (although this probably says more about me and my reviewing skills and it's not good! LOL).
As I've said before, this could also avoid confusion in that lord is normally a form of address and not a title as such. And it would be technically correct, so even readers that's up to date with their history knowledge would not be able to find fault.
Thumbs and big-toes crossed that you can come up with a simpler solution. I'll still keep this in the back of my mind - giving up is not an option! 
JR