Hi Elliot! I'm not very clued up as to "quality" of milk, but I really don't imagine that can be a problem after 17 weeks - a bf cousellor might know more about that one.
As for the centile worry, is your HV or GP worried about his weight gain? If not, then don't you worry either - they're only a guide and not set in stone.
Um, my take on solids is as late as possible - as close to 6 months or from 6 months. Your ds will be fine with that, it's more about whether you can stick out the "oh are you not weaning yet?" comments.
Milk has more calories than solids - you'd only be able to give him baby rice and fruit/veg mush anyway - you have so much more choice for interesting foods after 6 mths.
Ds had a late feed with expressed milk until he started solids at 5.5 mths. It was only once he started cutting down from about 8oz (about 2 boobsworth) to about 2 oz that I stopped waking him - that's when he didn't seem to need it.
After introducing solids I carried on expressing last thing at night before bed and kept the expressed milk in the fridge to mix with the mush. It also kept my supply up cos ds had dropped the late (11pm) feed around that time too.
Length of feeds - can't help you on that one I'm afraid! Ds was bf until last mth (he's 18 mths now) and he still took half an hour!! I waited for 10 min feeds too, to no avail!
Let him feed for as long as he wants until he starts that weird fluttery sucking - that's when he's finished and just using you as a dummy! If you cut him short then he might not have reached the hind milk which is the full cream stuff. I'm willing to be corrected on that one though.
Long post too!! hth