The scene of Cell you are referencing is not in the manga.
Yes it is.
You are forgetting that 60% of that creature’s height is just a long neck. A fairer comparison to Giant Piccolo (not Super Giant Piccolo-just Giant Piccolo) in terms of volume would be a large blue whale that has a mass of 170 tonnes.
Well Piccolo weight doesn't particularly matter because as he kid when he was much weaker he pushed this boulder while dug into the Earth.
His power level was about 100 so about 20x that of a regular human but hundreds of humans together wouldn't budge that thing. Bora's power level likely wasn't that much less, it could have even been more and like I said Tao's power level was greatly less than 180 but Bora couldn't budge his spear an inch.
No. Kaioken was flat out stated to double the battle power level.
In the volume I have it just says strenght but I suppose strenght could just be in general.
Additionally when Goku described Kaioken to Krillin (referring to just the regular Kaioken) he said that Kaioken multiplies ones strength, speed, etc by several times over. Not just 2
He may not have been referring to just Kaioken. He already knows there's greater than Kaioken x2 and Kaioken x4, x5, x10 etc is still just the technique called Kaioken.