quick/early : hayai you must know 早い and 速い arent the same word
For 早い and 速い, it's clear there is a different kanji, but if you start like this, you could also apply kanji to English words and some would have lots of different kanji. Just take a word like suject. Is that the subject of a sentence, a citizen, a topic... ? If English had kanji (it could still happen), it would increase dramatically the number of words, as every different meaning for a same word would be taken separately.

up/upstairs/top/on/above : ue joumen,kajou?
down/downstairs/bottom/under/below/beneath : shita kaika, soko, kamen ?
in/into/inside/indoors/interior : naka tonai、uchigawa?
out/out of/outside/outdoors/exterior soto kogai、sotogawa,uwabe?
Well, my Japanese is maybe far from native, but I can get help from my Japanese friends and relatives and I have asked many people (who could speak English fluently) about this issue. Just now, as I am writing to you, I care to ask and checked what you wrote and it seems that half of your translations are never used or don't fit in most situation. kamen and joumen are not in my dictionary, not even the Japanese/Japanese. My Japanese friends have never heard of it. Maybe you should give me the kanji. soko 底 indeed means "the bottom", but the bottom of the ocean or something like that. You cannot use it to say the bottom of the screen/wall, etc. You'd say sukuriin no shita, kabe no shita, etc. You didn't mention teppen for top. It would be the same ; you can use it for the top of a mountain, but not the top of the screen, etc. Kaika and kaijou works for downstairs and upstairs - I accord you these ones.
Then you come with tonai 都内 which means "inside the city", not just inside. I guess the compound -nai would work though. Translate it as uchi if you want, that's still the same word (same kanji). So uchi(gawa) and soto(gawa) indeed work for the inside/outside part, sorry to have missed them. However uwabe means "in surface or in appearance", which is not quite the same as outside/outdoors/etc (or add 2 more English words to the list). I don't want to look obstinate on this, it's just that I don't like so so translation.

What about words like ぴかぴか きらきら びしょびしょ etc. that dont have equally evocative English equivalents?
These are just onomatopoeia. I wouldn't consider them as words. That's true that English hasn't got so many of them, or of a different kind like "buzz", knock", "whizz", which are just imaged verbs or nouns. French and Italian have plenty of them and I find it natural to invent my own. I have plenty of them that are used by nobody else but me (and some relatives or close friends who hear me using them and adopt them). So, am I making my own language ? Sometime I wonder. But I realised that Japanese speakers could understand them as easily as French or Italian speakers (English-speakers more difficult for some reasons). It's just feeling expressed in sounds. It's over the language itself. (and I am still wondering why that doesn't work in English).

ookuno, kazoenaihodo, musuuno, taryou, houfuno, juubun, tappuri etc?.
All these have a different meaning from the one I have cited (though closely related).

ookuno : this one was alright > a lot of, much, many, plenty of, lots of, full of (ha, I forgot this one !), etc.
kazoenaihodo : innumerable
musuuno : countless, myriads of...
taryou : a large quantity/amount...
houfuno : a great deal of, in abundance, a great amount of, etc.
juubun : enough (occasionally with the meaning "plenty of")

That just make my English list longer, as there are still often 2 transaltion for each of them

I don't like it when you do that, because you give a false impression on other readers. If I hadn't come to correct you, people with a lower knowledge of Japanese would have trusted you as you seemed to know more than me when you said : How much Japanese do you know? I'm interested because it's going to have a big influence on how seriously I can take your points.Unless you're almost native standard then your argument that just because you don't know intricate ways to express yourself that they must not exist is difficult to accept, kind of like "..my guitar just doesn't have the right notes on it"

At least, I know that you aren't much more advanced than me, so you should refrain from making me look as if I had no idea what I was talking about. I don't claim that my Japanese is perfect, nor even advanced, but I care about precision and exactitude in translation, something you apparently take more lightly. That's all. No hard feelings anyway.