Never thought I would stumble upon this one here, but considering when it came out (born into a current of fluff and hentai in late 2007, early 2008) it has remained one of the most impactful and memorable Lucky Star doujin. I'm not sure if the ending was changed because of death threats (which I wouldn't rule out - and hey, he kind of was trying to censor their love!), but it was real nightmare fuel back in the day and I think it has aged well.
I kind of miss the days when I too would lean back and what threads grow, Kagamin. :(