1. Speed (available bandwidth) will be at least 2x lower on mesh-connected APs, as well as same part of root AP bandwidth will be busy by mesh traffic. As far as you are more concerned by coverage, and don't max capacity it must be OK to use 1-hop mesh connections for a few APs. Just take care that mesh link between APs has good signal -- so it is fast.
2. You need to enable mesh for all system (single checkbox), later you can disable mesh function on all APs, where it is not needed.
3. It's just one checkbox to enable, but than you want go through all APs and disable where mesh is not needed, and set mesh mode for APs which need it (Root or MAP, and choose uplink APs for MAP).
It will all work well and will provide decent 802.11n performance, even so ZD1012 and ZF7363 are very old devices.
Hope it helps.