Set inPoint with expression?

Or using 2 markers:

m1 = marker.key(1).time;
m2 = marker.key(2).time;

time >= m1 && time <= m2 ? 100 : 0

Expressions cannot control any property that does not have a stopwatch to set a keyframe.

You can use time and a simple if statement to animate opacity. If you want a layer to become visible at 60 frames into the timeline, you can use something like this:

t = time;
st = framesToTime(60);
if (t < st)
	0;
else
	100;

I use layer in and out points all the time to automate animations. This expression will do a 15-frame fade starting at the in point of a layer and then start a 60-frame fade out just before the layer ends. 

t = time - inPoint;
dur = outPoint - inPoint;
frmsIn = framesToTime(15);
frmsOut = framesToTime(60);

fadeIn = linear(t, 0, frmsIn, 0, 100);
fadeOut = linear(t, dur - frmsOut, dur, 0, -100);
fadeIn + fadeOut;

Maybe those ideas will give you some ideas of how to automate things and save time.

You may try to timeremap the layer and then implement a keyframe delay which you could control then via expression?

No, expressions can't change anything except the value of the property hosting the expression. Depending on what you're trying to do, you might be able to do it with time remapping and/or opacity expressions.

Scripting has access to in and out points (not sure if that helps).

Dan