Question:
any suggestions for flat, relatively warm (no freezing temperatures), half-marathons around October, January and April?

Answer:
I've run each of these. Very Flat.

Disney and Miami can be cool at the start depending on the weather.

PAL Half Marathon - Boca Raton, FL

Disney Marathon/Half Marathon
Orlando (sort of), FL

Miami Marathon/Half Marathon
Miami, FL

Try San Francisco US Half Marathon in October. Not totally flat since you go over the Golden Gate bridge but well worth the effort for the beautiful scenery.

The Long Beach Marathon and Half Marathon in October is supposed to be relatively flat but I haven't run this one.

Try the Carlsbad Marathon and Half Marathon in January but again I haven't run this one.

The La Jolla Half Marathon is very scenic in April but definitely not flat . Ran this one and will run it again.

All of the above are in California and have great running weather.

---

---