It wasn't two weeks, it was much more than that. And with Gido, Gon turned from a non-Nen user into a Nen user. So naturally, it's a huge increase in power. But with Knuckle, he barely changed. Being only able to strengthen his Ren, not to mention that he would fight Knuckle with 10% of his power due to the immense exhaustion. Doesn't matter who trained Kite, it really doesn't. And again, he was compared to Kite's physical strength alone. Which isn't anything to go by, because his physical strength isn't that awesome seeing as he uses OP weapons all the time.
We don't know that. Little Flower does a certain amount of damage that cannot be avoided with Ken unless the enemy is a top class Enhancer which Knuckle is far from. An explosion to the face can easily defeat Knuckle, EASILY. Unless he guards with Ko, which wouldn't negate the damage, would only minimize it. It's not a "Beat the weak." At this rate, I may say Gon's rock is a "beat the weak" because Uvo can take it no problem. Or Netero's Kannon is a "beat the weak" because the King laughed at it. Knuckle is by no means a powerful Enhancer so him surviving LF to the face with Ken is impossible.
It's not useless. I may just say that Knuckle's ability is useless because someone like Uvogin or Youpi can send him and his interests to hell in a single punch. Also, don't make it sound like Knuckle is so ungodly fast that Genthru won't react to him when Gon could react perfectly. If he tries to undo Countdown he will be wounded with LF or simply kicked because Genthru obviously has better hand to hand.
Dude Little Flower was blocked my Gon's Gyo. Crappy Gon who couldn't hold his Ren for half an hour, who's "Paper" is a tiny slow moving capsule, whose "Scissors" can barely cut through the stem of a leaf. The technique is useless against Potclean anyway. The critical problem is that Little Flower expends 2x amount of Nen for 1x the damage. In other words, for every X amount of Nen you are repaying Knuckle, you are depleting 2x the amount of your aura reserves - in other words,
your position is even worse, since the interest is incurred against your reserve i.e. If your reserve shrinks, then it's almost the same effect as incurring interest (with the minor difference that nen lent/damage taken from Knuckle is subject to the interest, while expended reserve is not). Following the same logic, abilities of a nen type that aren't the main school also fare badly against Potclean - it's like spending 2 dollars to repay 1 dollar of debt.
In terms of hand to hand combat, recall that Knuckle was several times moments away from
killing Gon in one shot. He's physically as strong as Kite. In terms of Nen, 30% of Gon's Ken (which is say, generously 1500 AP at G.I) blocked Little Flower with some damage. Let's say 40% is full damage prevention. That's 600 AP worth of damage. Let's very conservatively say that Little Flower's AOP, as a hatsu, is only 2x Gensuru's normal Nen output (it has to be
at least since he spends half the nen expended guarding against the explosion, which means if the AOP is less than 2x, he would do more damage throwing a punch). That's still 300AP, damage, 300 AP damage prevention, so 600 AP on each hand. With 1200 AP expended, only 10% was left for the rest of Gensuru's body. Conclusion: his Ken is only (roughly) 1333AP, even worse than Gon's. He would get the stuffing beaten out of him by Knuckle since Gensuru is somewhat like an Enhancer in that he must inflict damage via hand to hand attacks. His only hope is to use the bomb ability which takes a full minute to activate - which however btw in the context of Potclean, debits nen reserves IMMEDIATELY.
Knuckle's ability is indeed useless against guys who are better @ hand to hand than he is but then again Knuckle is atypically fast. He can probably avoid plenty of damage that way and the longer the fight goes on, the more buffer he has for taking damage.
edit:
Obviously the numbers are just close approximations. You can, for example, change Gon's Ken to 1800 and effective defense of Little Flower to require 60% of his Ken (twice the amount Gon used to cover his right hand). In this case, Little Flower will inflict 1080 AP worth of damage. x2 for both hands, and divided by 2 for the hatsu modifier, makes 2160 as 90% of Gensuru's Ken. 100% would be 2400AP. In reality, Gon's Ken is probably less than 1800 (say 1500) and Little Flower's modifier must be far greater than 2x. A conservative estimate would be 2.5x. In this case, Gensuru's Ken would come out as 1600 AP - still greater than Gon's hypothetical 1500 Ken, which would at least be consistent with the manga where it was stated that Gensuru's volume of nen was greater than Gon's.