a) 90 feet with no teak. Yet.
b) No more work than using a store bought two part cleaner. And I don't know anybody who doesn't think keeping a teak deck oiled isn't a PITA. That might be why you rarely see one.
c) To save money.
d) Good luck sanding down a worn teak deck with raised caulking with bronze wool.
Little or no extra work. You just mix and then scrub, like any two part cleaner. And making your own cleaner saves you and/or the boss some money. And the products you use to make the cleaner can be used for other things. Besides, many marine cleaners are just repackaged and way over priced house hold or institutional products.
Oil shouldn't need any grain to hold it to the teak. It should soak into the wood not sit on it.
And the smoother the wood the easier it is to keep clean. IMO, oil just attracts dirt and turns moldy looking.