In my experience:
1) Over long aging periods (typical for barleywines) hop flavour and aroma tends to fade. I've never tried the particular combination you have listed, but I wouldn't think there would be an issue. One of the fun things with barleywines is how they change over aging - I'd be quite interested to hear how your hop/malt combo works.
2) Efficiency tends to suffer, simply because the amount of water you'd need to get normal efficiency would then require an insanely long boil to concentrate to barleywine strength. The only time I get normal efficiency is when I do a weaker beer and then add DME. Even with 2-hour boils, by all-grain barleywines typically suffer a 5-10% drop in efficiency.
A good option is to give the grain a second wash, and use that for a lower-gravity brew. This is called a parti gyle, and is a great way to get the most out of a big-beer brewday,
3) No. You will need at least 2.
4) Mash time doesn't change with gravity; mash times are dictated by temperature and the mix of base malt vs. specialty malts. Mash as you would to get the desired degree of conversion and body you desire in a lesser-gravity beer.
Good luck & have fun!
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