The potential problems with removal of runner are dependent upon how the runner was installed. Some dearies actually tack the runner to the steps, tread and riser. Ouch, because removal leaves those holes to be filled and stained to match (very difficult to conceal). An alternative method is to use carpet bars which come in wood or metal, in an assortment of finishes. This is slightly more expensive but imo, also *looks* more luxurious. The carpet is barred at the back of the tread where it meets the riser; best practice has the runner tacked beneath each tread rather than slanted from top-front to the back of the next lower riser. Note that under-tread tacking also uses the most running feet of carpet, so the most finished appearance is going to cost. There are several other methods, so discuss intent with the installer before s/he does the work. One thing for sure, is to not use any sort of sticky-back tape: it either doesn't hold well, or holds too well and removel is labor-intensive. Because of the carpet pile height, if possible choose treads that are slightly deeper than usual so as to maintain fully safe depth.

If you are concerned about heat, LED doesn't even compare in any way whatsoever to the kind of heat you will get from halogen bulbs. Stay away from halogen if you want a cool room and plan to have lights on a lot. The LED strip lights are popular, but given your description, may not provide as robust a lighting layout as you are seeming to need here. If you are wanting lighting to mount underneath the shelving, you can check into the options from Task Lighting by Hardware Resources, made in the USA and affordably priced if you order from a dealer (always lower than MSRP): https://www.hardwareresources.com/tasklighting There is such a thing as a "lighting designer" that might be of help to you in a situation like this. You might want to check the resources for this in your area since you have unlimited options for what you can do.

Thanks I appreciate the comments. I guess I was thinking maybe a space the kids to bike around and a basketball hoop. Maybe instead of larger flat area I can see if they can install a climbing wall and a slide since I have the slope for it. Unfortunately, some of the shorter retaining walls are rotted out so I will need to do some retaining wall repair. I will also need to replace all the fencing and probably most stairs. It's going to be a large pricey project. :(

The old floor is in bad shape in other areas, it is 1.5" wide 60Y old hardwood installed on diagonal boards that have space between them. It is currently squeaking like hell. It has to go it was refinished partially (they did the hallway and the living and the rooms are still honey bee color) The nails might or might not hit that space so the plywood is there to make sure there is a flat support for the new hardwood floor, and to add sound insulation between floors