My Roadster 2 is very forgiving and I can't tell any difference between trimmers in or out at landing with regards to flare. Both scenarios have easily enough flare ability to stop on a spot.
Trimmers in has a slower rate of descent and a slower forward speed. Trimmers out has higher speed, because of its higher rate of descent!
Hence, the wings ability to flare is the same in either case. The difference between the 2 is the speed at which you pull the brakes to flare. Trimmers in (slow) requires the flare from hands up to full brake much quicker than with trimmers out. With trimmers out the flare takes longer as you have more speed to scrub off and pull the brakes more slowly, aiming to keep your feet just above the ground until you have slowed down.
If you have a clutched machine, engine on and at idle makes no difference.
Using power while landing is something to be done with great care. If you power off before you actually touch down then you swing back and accelerate towards the ground! Hence you need to power off high enough up that the swing back, then swing forward can stabilise before you reach the ground.