version 8 multiplayer desync'd and has been put to one side,

medieval has many limits that are not readily apparent even WHEN you bump into them,
(like the maximum number of castles and upgrades that can exist in a buildprod file)


if you are certain you have made no end of file mistakes or corruptions
then
i am certain a hidden limit has been breached

i am wondering have you tried version 8_MP

without new models
(maybe there is a max limit to number of models in total)

with a limited number of clans (say 8 only - same as a viking campaign)
(maybe 8 in an expansion is the limit)

with a strictly restricted set of character names
(maybe too many Lord "spare hero slot" 's)
(maybe too many names in total)

with only original king/ character names, start leader stats, text files
(maybe a value has been broken and that throws out the games database)