Speaker cable length.

@OP. You can do a simple experiment but it will require the two different lengths of cable. Use one speaker, and a mono source and then listen to the two respective lengths of cable and ask if you can hear a difference. Ideally, you would do this blind with someone else swapping over the cables.

It's not a perfect test because, theoretically, any difference could be magnified in the final stereo installation - but for the setup you are proposing i.e. not your main system, it's probably representative enough.

BTW since electricity in a copper conductor propagates at approximately two thirds of the speed of light there is not going to be an audible propagation delay factor in what you hear coming from the speakers.

However! As you have access to sufficient cable, why not just put the excess cable on the "short" run under the floor.

"Dude if they are different lengths your speakers will be OUT OF PHASE"

What a ridiculous statement.

One of those situations that can actually be measured if you really wanted to and had the equipment to do so. I’ve tried running equal length cable having both under the Obstacle with one looping back to the speaker when the distance was adequate to take up the extra cable. Satisfied my what if thinking, but that was all ! Did  A/B testing with several audio people using on one side a 13’ shorter piece.., “0” audible difference. The equipment is a large multiple speaker system and very revealing, the DIY wire being of good quality with complementary fittings. Would seem to be an either way personal try it situation, my guess won’t make any difference.

Cheers

Phase has nothing to do with cable length.