To echo a bit what fishmaster also did, I think many of the answers will vary based on the type of set up you have. If you are going with a high tech, heavily fertilized, fast growing, nutrient burning tank, you'll get one set of answers. If you are going with a soil-based, El Natural, fish poop for ferts, lower light setup, you'll get something else.
#1. Most of the plants I have tried seem to grow quite well for me, and I've only used substrate ferts very infrequently (typically for targeted plants like crypts and swords). Many on these boards use no substrate ferts. So, as many of us are on the higher tech end of things, in a high-tech setup, the root uptake of nutrients is less important.
#3. I'm sure there is some very small loss, but I would not call it a 'noticeable degree'. As to 'does it affect slow glowing plants', what is the 'it' referring to?
#5. From all I've read, aside from a few plants when will only tolerate either soft or hard water, most plants (and fish!) are incredibly adaptable. As to benefits, let's see. What plants/fish want most is stability. Thus, if you can use your local tap water without modification (and the local tap water doesn't fluctuate too much!), then its use should provide the most stability during water changes. So, unless you have specific reasons not to (e.g. kh<3), then go with your tap water.
#6. Until quite recently, I couldn't see more than 10% of my substrate. So, I never touched it during a water change. I do have quite a collection of MTS, so I'm hoping they help keep the substrate stirred up somewhat to prevent anaerobic conditions. This is one 'lesson' I haven't had to wrestle with yet.
#7. I believe that plants can more easily process the 'N' in ammonia and nitrite more easily than they can the N from nitrate. So, I would say they are good for the plants. Now in excess, the fish may disagree!
#8. There are lots of posting on the boards that talk about the reflectivity of different materials (e.g. white versus black paint). I seem to recall that white paint had a rating in the 70+% range, while the fancier reflectors (e.g. the AHS variety) were in the 90+ range (and I could be off on these numbers). Based on enough folks commenting about how much brighter their tanks appears (though, this is the visible spectrum, which the plants don't really care about) after going to AHS reflectors, I would think there is a noticable difference. Now, not knowing for sure how you quantify 'big', I can't address that part.
#9. From my limited readings on the siesta, I don't believe most on these boards seem to feel it is really of any benefit.
#11. I answered this question similar to how fishmaster did above on another board recently. Then I asked Roger Miller to provide his comments. His response what that though activated carbon (AC) will initially take out useful nutrients, AC tends to clog very quickly. Once it does, it can actually become a useful media for growing bacteria useful to the tank. I know of a few highly respected aquascapers who actually pore a box of activated carbon into their substrates for that very reason. Initially it can help soak up excess ammonia and the like, but after that, it becomes beneficial. So, I guess I'd now say that as long as you don't replace it too frequently, it shouldn't be a problem. Then again, if you're putting it in the filter path, you are likely doing it to suck up stuff, so why leave it there for too long? Personally, I don't use it in my plant filled tanks.
#12. I can't give you specifics, but (once again, data from Roger Miller) when certain nutrients get to very high levels it can block the uptake of other nutrients and thus become detrimental.