I am afraid that this unit does not have a power-saving mode.
You won't be able to adjust the brightness.
Approx 6.5 cms
This unit can be installed on the wall.
That is correct, it is bright as one Selene lamp.
The solar panel can be only angled up and down and not rotated.
It's hard to say and subjective. You'd really need to test it see. At 44 Lumens this is the least bright spotlight we have.
This really depends on many factors, this light is 44 lumens, our spotlights go up to 140 lumens. You would also need to test if one or many lights have the desired effect you are looking for.
Yes
The Helios is rated at 44 lumens.