As far I know is up to The Court to decide if need an expert report or the evidence provided(photos) is enough.If the court asks for expertise and you win the case you can ask refund of expertise.
For claim money back you must pay the invoice in full.You can't stop payment, and for claim...
I have the same issues but just with white one, hard to fill in,hard to archive the nice finish,but start this morning with antracite and work really fine, easy to apply. Seems the white one have different sand and not work in the same way.