Is optimal landing flare related to ground effect?
It depends on the type of landing.  If you are doing a normal landing on a normal runway, the round-out and flare are required steps to put you in a position to be in level-ish flight just above the runway (the roundout), and controlling the touchdown by trading inertial energy for increased lift by pitching as you feel for the surface (the flare).

Ground effect has an effect on the flare phase by giving you more energy margin if the ground effect is strong.  So a low wing airplane with large flaps with a strong ground effect will have a lot more support from ground effect than a high wing airplane with small flaps that has weaker support from ground effect.  If ground effect wasn't there, it wouldn't really change how you land too much, except you may tend to carry power into the landing more, or arrive with a bit more energy by increasing the approach speed a bit.  In some airplanes the ground effect is much more pronounced than others (they like to float).

There are landings where you don't really flare at all.  In an extreme STOL approach, you fly to the surface already in the landing attitude, controlling descent rate with power and maybe little pitch adjustments only as required for speed corrections.  If the engine quits during such an approach close to the ground you are out of energy, already "flared" and will land hard and you live with that risk if you're doing that sort of thing.

And of course, there is no flare in aircraft carrier landings.  You fly a constant descent into the ocean, and the carrier's deck just gets in the way of your descent.