Yeah, ithe first Solaria/LN thing wasn't a total realm merger because Malus left one rural region behind. Therefore it's legit because one guy with a single rural region didn't get to go with them. But when Terran leaves a city behind, it breaks the rules.
IIRC, Malus left behind a (small) Duchy, including a stronghold. And the Duke was pretty damn pissed.
Terran leaves behind a city, because
it is mechanically impossible not to now that I've fixed the bug, and
explicitly plans to have D'Hara conquer that city.
This
is a material difference. Intent counts.
This case is bogus. The rule prevents a "merger of equals". In what reality are Terran, a besieged dying realm of three region, and D'Hara, a thriving healthy realm of 16 regions (or so, can't check right now...) anywhere near equals?
A "merger of equals" is not required to trigger the no-realm-mergers rule. Only a friendly merger.
This is planned between the rulers, it is not a surrender, and it is clearly a violation of the realm merger prohibition.