What I mean is they used a Newgrage as the core, the starting point. and they built the station and yard up around it, using a decommissioned Newgrange as the starting point to built the Lum yards.
Imagine if they had started with the Newgrange and then built up and around her to add dry and 'wet' docks, as well as other facilities etc, but it all started with the central core of the newgrange that's probably buried under the changes made and the yard that's built around her save her drydock and her command and control facilities/reactors.
If Lum was something like this
Then the Newgrage would be like the central core or one of the side arms or something. That's how I kinda picture it anyhow, she's not the main yard, but the Ravens built the yards up around her.
We don't know what the Lum yards looked like (or something like the Titan yards which were bigger) but it could be something like Scorpion yards from BSG
With them being a huge sprawling thing built up and out, and buried somewhere in there could be a Newgrange or two they built the yards up and around.
And I like your idea for the Free Guilds, confined by law to have no 'military' equipment, and confined within the SM System although you can bet that their facilities have some weapons, they don't want any Bandits to try anything. But then again to try anything in Strana Mechty would probably be suicidal. If they do venture out of the SM system its ONLY at the behest and 'request' (really an order) of the IlKhan which they can turn down at the risk of irking the IlKhan and so no ilKhan since Nickyboy has done so.
So instead they can 'loan' their ships out to a Clan, and its up to that Clan to escort and protect them from any Trial, because if they are lost, even in a Trial, then the Guilders will expect to be repaid in kind "You lost two Mules, you will give us two Mules." kind of thing.
We don't know how many Newgranges escaped with Kerensky, but with 200 built and 45 left in the Inner Sphere, I'd say there was probably at least 60 - 80 of them who went with the General. Now how many of them survived the Pentagon civil war is questionable, but probably not that many until Special K came along with the Clans and brought 'order'. But the Civil war was unkind to these hugs ships, that probably would have found new use as converted orbital habitats and the like rather than repairing warships. So say at best 20 survived, with say...6 - 8 under the Guilds either over Strana Mechty itself or in system, the rest divvied out to the clans where they were either put to use or mothballed.
Also I find it impossible that there's no other type of Yardship so if there was any other class, there might be some of those still around that did survive would probably be smaller and less capable than a Newgrange class and thus might find use with less naval minded Clans.
RE Colossus and CargoKhan, the Colossus is a military dropship and not many survived the fighting in the Pentagon Civil war, but why would you need so huge a transport dropship when you've got mules etc. The Cargoking was brand new at the end of the Star League and the Clans probably have its plans somewhere, it was built for the SLDF so someone's probably got the plans.
With regards to who actually 'owns' the surviving Newgranges, I'd say that some are spread amongst the Clans.
Ravens - 1 used to kickstart the Lum yard, another in service or mothballs as a standard vessel
Star Adder - 1 ship
Wolves - 1 ship in Mothballs as Kerensky's chosen and faves they get a shiny toy that is now in orbit round a distant star slowly gathering dust.
Sharks have one, Bears one etc etc
but the majority of them would be in the Strana Mechty system for use by ALL the Clans as neutral ground, probably overseen by the Ebon Keshik who will crack down HARD on any Warriors doing shenanigans aboard them because they are neutral ground and no fighting is allowed. One such ship in a disarmed and coreless state could even operate as the Free Guilds 'Capital' so to speak.