Consider a scenario where an airplane is operated on an ETOPS 180 route where the ETOPS alternate is open, available and above minima, but there is terrain between the route and the airport which makes it absolutely unreachable. If, in that scenario, an engine failure or decompression alternate is available but more than 180 flying minutes away, and enough fuel is carried to reach it while satisfying the terrain clearance requirements, would that flight be legal (not: practical, safe, desirable - only legal!) according to EASA/FAA regs? In other words, is ETOPS satisfied solely by looking at range circles around available airfields, or are there additional considerations required (not: prudent, desirable - required!) by regulations?
No, ETOPS is not satisfied solely by range circles, but (among other things) by the "time needed to fly to that airport".
See the requirements of 14 CFR 121.633 (emphasis mine):
(a) For ETOPS up to and including 180 minutes, no person may list an airport as an ETOPS Alternate Airport in a dispatch or flight release if the time needed to fly to that airport (at the approved one-engine inoperative cruise speed under standard conditions in still air) would exceed the approved time for the airplane's most limiting ETOPS Significant System (including the airplane's most limiting fire suppression system time for those cargo and baggage compartments required by regulation to have fire-suppression systems) minus 15 minutes.
Paragraph (b), for ETOPS beyond 180 minutes, gives the same "time needed" requirement.
So, if a scenario existed as the question describes where "there is terrain between the route and the airport which makes [the ETOPS alternate] absolutely unreachable", then that flight could not reach that alternate in the required time (since the alternate is not reachable), and the flight could not be dispatched listing that alternate.
This isn't ETOPS specific, but is related to high terrain. When overflying terrain that limits your ability to reach your destination or destination alternate you plan additional alternates called "drift-down" alternates. The flight plan is then segmented into drift-down segments that determine which alternate you will land at if needed for that segment of flight.
The drift down information will be part of the dispatch release and assures that from any point on your route with an engine failure you can meet obstacle clearance requirements while drifting down toward a suitable alternate airport.
Routes I flew between the US and Mexico often had drift-down alternates as there were areas on many of our routes that we could neither return to our departure airport or make our destination if we experienced an engine failure. In those cases we'd reference which drift-down segment we were in and head toward the alternate for that segment.
The regulatory basis for the drift-down procedures and alternates is 14 CFR 121.191. The synopsis is that flight with a failed engine must be able to proceed to a destination or alternate from any point along the route while maintaining specific terrain clearance requirements. The "drift-down" alternates must meet the same requirements as normal alternates (weather, suitability) and must be present in the dispatch release.
Combining this with ETOPS, the drift-down alternates would have to be within ETOPS range in addition to the terrain drift-down requirements.
§121.191 Airplanes: Turbine engine powered: En route limitations: One engine inoperative.
(a) No person operating a turbine engine powered airplane may take off that airplane at a weight, allowing for normal consumption of fuel and oil, that is greater than that which (under the approved, one engine inoperative, en route net flight path data in the Airplane Flight Manual for that airplane) will allow compliance with paragraph (a) (1) or (2) of this section, based on the ambient temperatures expected en route:
(1) There is a positive slope at an altitude of at least 1,000 feet above all terrain and obstructions within five statute miles on each side of the intended track, and, in addition, if that airplane was certificated after August 29, 1959 (SR 422B) there is a positive slope at 1,500 feet above the airport where the airplane is assumed to land after an engine fails.
(2) The net flight path allows the airplane to continue flight from the cruising altitude to an airport where a landing can be made under §121.197, clearing all terrain and obstructions within five statute miles of the intended track by at least 2,000 feet vertically and with a positive slope at 1,000 feet above the airport where the airplane lands after an engine fails, or, if that airplane was certificated after September 30, 1958 (SR 422A, 422B), with a positive slope at 1,500 feet above the airport where the airplane lands after an engine fails.
(b) For the purposes of paragraph (a)(2) of this section, it is assumed that—
(1) The engine fails at the most critical point en route;
(2) The airplane passes over the critical obstruction, after engine failure at a point that is no closer to the obstruction than the nearest approved radio navigation fix, unless the Administrator authorizes a different procedure based on adequate operational safeguards;
(3) An approved method is used to allow for adverse winds:
(4) Fuel jettisoning will be allowed if the certificate holder shows that the crew is properly instructed, that the training program is adequate, and that all other precautions are taken to insure a safe procedure;
(5) The alternate airport is specified in the dispatch or flight release and meets the prescribed weather minimums; and
(6) The consumption of fuel and oil after engine failure is the same as the consumption that is allowed for in the approved net flight path data in the Airplane Flight Manual.