The blocks are nice and cheap. Mix with perlite and go. They will have nothing in them tho so at this stage you will need some
calmag. I would definitely transplant tho. That soil is probably sure hot.
Ro water is fine in Coco. Will need to pH up tho at the end of mixing nutes (later when the plant is a bit bigger. No nutes other than
calmag at this stage). With soil ph is supposed to be around 6.5-7 I think but with coco 5.7-6.0 is perfect. Once you get the coco mixed up, mix a batch of to water with
calmag at 1/4 recommended strength and just keep pouring it through the coco mix to wash out anything in it. Transplant only after you've washed the coco a lot. When you water, go until you get a bit of runoff. If you wanna know when to water, the top of the coco can dry out and the pot can lighten up a bit and it's time to water again. You can overwater Coco but it is tough.
As far as your light goes, stretching is far prefered to drooping. Your light had a recommended hang height at the veg stage. Mine is 24" but 36" worked just fine for me. Too close and you're introducing heat and more light than they can process.
Hope you can pull this grow off. Hate to see a fellow medical patient suffering. Good luck!