I think the basic Idea behind you question is in #1 is should you
retract the flaps to try to extend your glide. The answer is it
depends mostly on how high you are. Below about 400 feet don't mess
with them. You will not gain enough to be significant and you should
be concentraiting on flying the airplane. Above 400 feet retracting
them can have some benefit, The higher you are the more it will help.
As I recall in the 172 going from 30 to 20 would give a signficant
drag reduction. From 20 to 10 does not do much and from 10 to 0 does
even less.

Best answer is find a willing flight instructor and go out and try it.
I do scenerio's like these all the time with students and on flight
reviews.

For Senerio #2, If landing ahead is no longer an option then get the
airplane configured as close to the Vx configuration and speed as
possible. I don't recall what flap setting this is for the R model 172
as I recall some 172's recommend 10 degrees of flaps and other say
that the 10 degrees doesn't help over an obstacle. I would be inclined
not to mess with them as long as they were 10 degrees or less.


Brian
CFIIG/ASEL