24V Circuit Protection - Fuse, TVS Diode, RC Snubber
Circuit protection isn't my forte but since I was involved in the development on Relay for Intermittent Wiper Function I will chip in.


  Is the 100mA PTC fuse appropriate for the circuit?


It seems appropriate.


  Is the TVS a good idea or unnecessary with the LM2931 already having 60V load dump protection, -50V reverse transient protection, internal current limiting, and temporary mirror-image protection?


I don't know.


  Is the 6.3A fuse appropriate? (The motor power from M_LO and M_HI are on a 20A fuse, much too high for the PCB). The motor can pull over 2A at start and about 1A while in operation.


If your measurements are correct it's fine. The 2 A start current sounds a bit low. What happens if the wipers are loaded with snow or leaves?


  Are the emitter resistors (R4 and R8) appropriate? I did not have them in the original circuit but believe there should be some value there to limit emitter current to ground.


No. 


The datasheet says that the relays are 53 mA at 12 V. The most you can get through 1k at 12 V is 12 mA so the relays won't pick up.
They will cause the emitter voltage to rise up to the Tiny's output voltage and will cause Q2 and 3 to be only half-on. You have designed a kind of negative feedback current limiter.


Just bridge them out or put in wire links if you've made a PCB.


  Are the snubbers necessary for a 28V/2A motor relay? Selection of snubber components acceptable?


They can only help. The motor is inductive and they should protect the contacts - although you've only snubbed the NO side.


  Would any components apart from the TVS diode and relays need to be substituted when building for 12V (~14V) use rather than 24V (~28V) use?


Looks OK.



The only other comment I have is that D6, 1N4148, is rated at 100 mA (from memory) and might be a little puny for the relay. A 1N4001 (1 A) might be safer for D6, D4 and D5. Take others' advice on that.

I have not checked the relay logic matches the design worked out in the original question.