That, as far as I also understand, is correct.
I don't think so. A Reach weapon doubles your reach. So, it'd be 10 base, doubled to 20, then +5 for Lunge. 25' total.
A large-sized creature has a normal 10-foot reach.
The Lunge feat adds 5 feet to the reach.
A reach weapon doesn't simply add 5 feet to reach. Most reach weapons double the wielder's natural reach. So, a Large character wielding a reach weapon of the appropriate size can attack a creature 15 or 20 feet away.
So, your Kusarigama wielding large Ninja friend can lunge and hit targets up to 25 feet away.
Would this mean that a Tiny creature wielding a reach weapon would still have a reach of 0? Would it need the Lunge feat to be able to use a melee weapon on an opponent 5' away?
By the rules-as-written a tiny creature has a reach of 0 regardless of weapon. Doubling a reach of 0 is still 0.
Personally, I would rule that since typically reach is the same size as your space (5'x5' medium creatures have 5' reach and 15'x15' huge creatures have 15' reach) it would be reasonable to allow a 2.5'x2.5' tiny creature to have 5' of reach.
Might be fun to fight a pole arm wielding imp. Doubt it would be game breaking. Diminutive and fine creatures are still out of luck.
