Time for an update! Thanks to all for the suggestions. I have resolved many of the problems I was seeing. I learned a valuable lesson - never trust PO wiring.
Some specifics:
Horn not working - turns out I must have breathed near the connector to the horn in engine compartment. Just pushed it in and now the horn works (also identified I am missing the driver side horn).
Brake lights - isolated the issue to turn signal switch. The arm is very ‘wobbly’ and after running my fingers along all bullet connectors and securing I wound up putting some pressure on the column and brake lights started working. I will pull it this weekend and have a replacement on the way.
Turn signals - FR, FL and RR all work. The RL will flash only once when arm is moved, so I traced the wiring back checking resistance and voltage out to lamp. It all seems solid, so assuming this will also be resolved by new turn signal switch. Voltage levels are also now even and lamps bright to same level. I wound up disconnecting all wiring up front from the blocks, and just doing it myself making sure that the circuits were correct per wiring diagram.
Hazards - resolved by replacing burnt out inline fuse under cowl.
On to reverse, license plate frame and interior lighting!