The right way to solve #1 at least is with a computer program, but by hand I hereby offer
A not-very-reliable dictionary file found online has
overnumerousness (16) and nonerroneousness (16), which both seem pretty dodgy to me; nonsensuousness (15), nonvenomousness (15), and overnervousness (15), which seem no better (note: the last has already been posted in another answer); convexoconcave (14), curvaceousness (14), nonamorousness (14), nonconcurrence (14), nonconversance (14), nonnervousness (14), nononerousness (14), nonvacuousness (14), overcommonness (14), overnarrowness (14), oversevereness (14), unnumerousness (14), unsensuousness (14), unsonorousness (14), unvenomousness (14), of which I'm reasonably convinced by at least curvaceousness (14) and nonconcurrence (14) though some of the others seem possibly OK.
In other news,
my team of sex educators has been working hard to prepare a curriculum to supplement school sex ed with extra ... practical work ... in the holidays. Good news! We just got approval to go ahead with the project, so we're definitely going to do it. So: our new success means sure summer sex course. But I bet it isn't difficult to make a much longer coherent sentence than that. For instance, some of our vacation students who were feeling gloomy earlier now report that summer cares are now no more as sex success means our amorous new women can soon arouse us (74/18). With just a very little kinkiness they could be mouse women and perhaps it might then be zoo sex success but these feel kinda cheaty. Similarly, they could be vexed summer cares (abusing "vexed" in the same way as the stock phrase "vexed question" does) but that also seems cheaty.
Moving from sex to death,
my zoo employs a team of first-rate medical scientists, who have been working on a terrible problem that made us all sad. Our zoo had a rare kind of mouse, but unfortunately the harsh sunlight at this time of year repeatedly gave them skin cancer. But now success succours summer woes: new wax can cure some cancers -- mouse sarcoma curse vexes our morose zoo men no more (94/20).
It turns out that this research (unlike most research on cancer in mice) actually applies to humans too. That's a real relief -- we and our partners had been afraid to indulge in all the outdoor frolics this season demands. But now we can stop worrying: cancer source -- summer sun -- can never more scare us as new wax cures sarcomas -- erases morose nervousness so now we resume our amorous romance: reassurance means curvaceous nurses arouse amorousness anew (170/31). Why nurses? Well, we're feeling so grateful to the medical profession...
Of course there are other ways to deal with death, which may not be so agreeable. For instance (I am not going to try to make this terribly long; it's just for amusement):
nervous coroners scream as necromancers rearouse carcases.
But, while keeping with that magical theme, let us finally return from death to sex again. Warning: this one is a little bit explicit and, worse, it uses punctuation that goes above the median. No numbers; I'm just playing with the sounds one can make with this repertoire of words.
Those naughty necromancers are ravishing old nuns and making them feel young again, now: senescence evanesces as sensuous sorceresses' excess caresses access ancresses' recesses.
Finally -- still nowhere in the realm of maximum length, so I shan't bother with spoilerification, and I scoff at the no-repetition rule here -- let me quote a couple of famous lines:
Pascal: enormous cosmic cavernous vacuousness' unsonorousness never ceases -- scares me
Shakespeare: names are vacuous: we name roses so -- a new name never removes savorousness