As addressed before, glassy water landings are in a class by themselves. Highly desceptive, depth perception is nearly impossible.
Depth perception is not nearly impossible on glassy water " it is " impossible.
An approach to landing should be made at minimum sink speed, constant descent until water contact.
That is correct in the case of an engine out in a single engine airplane with glassy water you should aim for minimum sink speed...and pray.
With angled light there's a greater chance for shadowing, but if it's truly glassy water, then having an idea of the surface winds and planning accordingly is the best bet.
I am not exactly sure what you mean by that last part.....if there is a surface wind there will be ripples on the surface of the water and it is no longer a glassy water surface because once you get any wind there will be at least ripples and the surface then becomes definable.
Landing on water with even small waves/ripples is no big deal as far as judging height goes for the flare altitude.
When doing initial sea plane training I have the pilot being trained do all approaches and landings with the throttle/'s closed from 200 feet until they are proficient at flaring and landing without using power.
I have been training sea plane pilots for fifty years and have never had any problem with them learning to flare and land power off...it is just a matter of getting the proper sight picture and changing the attitude at the proper height above the water.
A lot of my clients are heavy jet drivers and at first it is a bit strange to them but they soon learn to land power off and then I let them use power as they see fit.
During the last fifty feet I count down the height to the surface (From 25 feet to five feet I count down in five foot increments, from five feet to touch down I count in one foot increments. ) this aids them in judging height and the closure rate and the sight picture that is required to approach, flare and touch down in the proper attitude.
I use a camcorder and we de-brief on a TV screen.
When they screw up an approach/flare/landing I stop the video and I have them use a laser pen and put the dot where they were looking at that point in time and ask them what they were thinking at that moment with regard to height above the water......it works like a charm as a de-briefing tool.
Generally they are looking to far ahead and their ability to judge height is affected.
Ocean swells are a subject that really would be difficult to go into in any depth on a private pilot forum as there are so many variables and methods of judging their presence and behavior. I have seen them with around four hundred feet from crest to crest and deep enough in the trough that all you can see is a wall of water in front of you and the sky.