There could be some reasonable explanations for SOME of what you report...It's not unusual to find "foreign" bodies in mortar- leaves, sawdust, nails, maybe a fag butt. But this shouldn't be pervasive! It's not acceptable in a render mix, but can be found in mortar and concrete mixes, where housekeeping can be poor. A spare earth tail may sometimes be left out in a wall for connection to a sink/ metal plumbing or other grounded device which is not yet fitted but may be in the future. Flues should certainly be cleared of rubble or debris, especially after building work. If in doubt go straight back to the installer and voice your concerns- if your not happy with the explanation say so. A competent installer should be happy to stand over their work.