I would replace the pump as it isn’t a safety risk.
Just give written advice that the unvented unit plumbing has been done against regs and manufacturers instructions and must be redone.
No point in looking for and stating every fault as not your responsibility.
Seems weird that a tundish has I think been used on heating discharge, yet unvented pressure relief has been joined straight into D2.
That Pipework looks like the contents of the scrap bucket in my van!!
I would go for it. Advise them about the other stuff. You didn't install it so just bring it to their attention and when you give them the invoice, add a footnote saying the same thing.