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"сказал что? - неправду."
"сказал много чего? - неправды".

"difference between неправду говорили and неправды говорили"

It's just grammatical case.

By the way, it could be tricky for non-native to understand what is "неправду говорили" about and relation of it to "виднее наверху".
Some time after death of Stalin he was widely criticized by next...

"what's the difference between видней and виднее?"
no difference

"Да" introduction particle. In this case it can be traslated as "And..." or just omitted.
"всё" before verb can be translated as "all the time". That is action of verb lasts long period of time....