One-step is actually quite good as a mild strength sanitizer as well as a cleaner, but as has been noted in other threads, it may not be quite as effective as commercial equipment sanitizers (it kills micro-organisms with hydrogen peroxide, which is generated as the powder is mixed with water), and so the manufacturer doesn't want to make claims that are unsubstantiated. They also do not want to run through the full gamut of testing that would be required before the product could be certified as a sanitizer (expensive, and there is no guarantee of success in the testing).
Since I re-use commercial wine bottles extensively, I will tell you that I have had excellent success with my "two-step" process. First I soak the bottles in a one-step (or similar percarbonate cleaner) solution overnight, then I brush those that appear to have obvious gunk stuck to their insides, then I rinse with a solution of metabisulphite, then I bottle. Occasionally I will use Star-san instead of the metabisulphite (depends on how much of both that I have on hand), but the process is always the same for me. Clean with a percarbonate based oxygenating cleanser, then sanitize, then bottle.
#8 corks can be used in standard 750 ml wine bottles, but they will be at the limit of their elastic expansion capability, and as a result they will not last as long as #9 corks, which are more tightly compressed when they are in the bottle necks. So if you don't plan on keeping this mead for very long, you can use your #8s, but you will be better off with #9s in the future. Also, as you noted, the hand corker will be very difficult to use with #9 corks so you will eventually want to get a floor corker.
If your mead is dry then you will only need to add metabisulphite (and even that addition is a matter of personal preference - many dry meads have been successfully bottled with no chemical additions and they keep well). However if you do have residual sweetness (or you intend to backsweeten) then sorbate plus metabisulphite are recommended.
Since I re-use commercial wine bottles extensively, I will tell you that I have had excellent success with my "two-step" process. First I soak the bottles in a one-step (or similar percarbonate cleaner) solution overnight, then I brush those that appear to have obvious gunk stuck to their insides, then I rinse with a solution of metabisulphite, then I bottle. Occasionally I will use Star-san instead of the metabisulphite (depends on how much of both that I have on hand), but the process is always the same for me. Clean with a percarbonate based oxygenating cleanser, then sanitize, then bottle.
#8 corks can be used in standard 750 ml wine bottles, but they will be at the limit of their elastic expansion capability, and as a result they will not last as long as #9 corks, which are more tightly compressed when they are in the bottle necks. So if you don't plan on keeping this mead for very long, you can use your #8s, but you will be better off with #9s in the future. Also, as you noted, the hand corker will be very difficult to use with #9 corks so you will eventually want to get a floor corker.
If your mead is dry then you will only need to add metabisulphite (and even that addition is a matter of personal preference - many dry meads have been successfully bottled with no chemical additions and they keep well). However if you do have residual sweetness (or you intend to backsweeten) then sorbate plus metabisulphite are recommended.
