Really this is a
girder truss shoe, not just a hanger (but most people call them that)
Zurich Solid Foundation states
"Mono-pitched trusses are fixed to girder trusses by metal shoes. The bearing of mono-pitched trusses onto the mild
steel proprietary girder shoe should be confirmed with the
roof designer before site installation is attempted."
Also nailing into end-grain is not really a good method of fixing timber - let alone trusses
I'm not sure that the other items you mentioned would make the roof structurally unsound. They may be just quality issues, which could be outside of the BCO's remit
Get a cost for the remedial work, and contact the LA explaining the situation and that you feel that the inspection regime was inadequate and that you are now faced with a cost of £x amount for remedial work and the truss manufacturers warranty to be valid. Ask them to respond with their opinion within ten days.
Your recovery options are then with the Local Government Ombudsman, and/or via a civil claim (small claim).
If you are going to try for compensation, then you would need an expert report from either the truss manufacturer, or other suitably qualified person