I think u should get schluters take on the problem.
If they say u have a correct installation minus the boards, ie u've left the cable back from the kickboards the correct distance and that it is usual for a decline in temp at that point on the floor then u'll have an arguement!
The insulation board wouldn't make a difference to the heat dropping off at the edges as far as I can see.
It'll just take longer to come up to temp.
The heat will radiate a certain distance from the cable, so if each cable let's say radiates heat over 50mm, then beyond that it will cool, so at 75mm the edge will be cooler.
On the other hand, if they say u can take cable up to edge of kick boards,
Then u may have a problem.
As far as builder taking responsibility, well you're the Proffesional floor layer, he is not!
I would imagine that in a legal situation, u'd be the one held accountable because you're the Proffesional.
Pretty much in the same way a disclaimer written by u and signed by a client for u to cut corners isn't worth the paper it's printed on.