Another issue nobody noticed ( ):

IT'S NESTED TRIGGER

INSTEAD OF UPDATE trigger makes UPDATE to the same table and fires itself.

I don't see in trigger any mean to stop this endless loop.

But in general view:

wouldn't it be easier to have separate table to log changes:

ObjectId, PK_Id, DateRecorded, UserId.

Triggers on monitored tables will just add records to this table.

Easy and reliable.

And you don't need to care about freaky datatypes (did anybody think about user defined ones?) in monitored tables.