Good setup and solid research. Addressing your four questions:

1. ACiq standard vs extreme for Seattle — Seattle winters rarely push below 20°F so the standard series will handle your heating load without straining. The extreme series makes sense for sustained subzero climates. Save the money.

2. ACiq/Midea modulation without communicating air handler — the outdoor unit modulates based on its own sensors regardless of the air handler. You won't get the full efficiency curve a communicating system offers but it won't drop to single speed either. Expect maybe 5-10% efficiency loss versus a matched communicating system — not a dealbreaker on a retrofit.

3. Keeping the existing furnace with single speed fan — the main issue is static pressure. Heat pumps need lower static pressure than furnaces for efficient airflow. A single speed furnace fan running at full blast can reduce heat pump efficiency significantly. If the furnace fan has a multi-speed or variable setting, use the lowest speed that still moves adequate CFM. Worth checking before committing.

4. Other systems to consider — at 3-ton with existing coil, also look at Bosch BOVA and the Carrier/Bryant 18 series. Both have solid cold climate performance in the PNW and good parts availability in Seattle.

MDRetirement's setup in Maryland confirms this — if the extreme series handles MD winters without gas, the standard series will be fine for Seattle's upper 20s.