(1) your question is very confusing.

" (A) If “clearly evident” is redundant ...

(B) ... what word or phrase suggests being indisputable but not readily apparent?"

part A has no connection to part B.

i. 'clearly evident' is not redundant. (most of the answers to the linked question are utter crap.)

("clearly evident": you could also have "slightly evident" "evident if you turn on the xray machine" "sometimes evident" (depends on the tides) "barely evident" "happily evident" and so on. end of debate. you might as well assert that "tall" or "strong" can not be modified: sometimes strong, extremely strong, a little strong, etc. on here you often get questions with hopelessly silly answers, and that was an example thereof.)

ii. if 'clearly evident' was redundant - so what? it is utterly normal in English to emphasise an otherwise logically non-modifiable word with a emphasiser. (You may as well state "In English, that's not a 'redundancy', it's just what we call an 'emphasizer'.)

iii. completely setting aside i and ii -- your "A" has no real connection to your "B". You are simply asking:

"What word or phrase suggests being indisputable but not readily apparent?"

Am I correct?

Another way to phrase what (I believe) you are asking for is, say, "What word means being definitely true, although not obvious or simple to demonstrate to be true."

(Note that this would commonly apply to say certain mathematical theorems.)

Funnily enough, there was just a meta discussion ... "regarding single-word-requests, should we make more use of the answer 'oh, there is no such single word'?"

Quiet simply *there is no single-word for "definitely true, although not obvious or simple to demonstrate to be true".

Some phrases that come to mind in related situations are: "not immediately obvious", a "hidden truth (not exactly what you mean, but similar), and the witty form: "seemed obvious but only to blah. (So, "The logic proving Fermi's Fourth theorem was obvious .. to Fermi.") A useful word is convoluted ... so, something like "the proof is convoluted" or perhaps "it may look false to you, but it fact it is true: the proof is convoluted".

Watson would simply say in a book "the blah blah is true, although difficult to deduce." Or again, he would use the common form: "It was obvious .. To Holmes," which is a witty way of saying "the proof would be convoluted to anyone else..."

If it helps, I'm going to go ahead and say the closest thing to a single-word answer is convoluted ... but read the caveats

TBC that doesn't stand alone, you'd use it in a short phrase, so, "it's true, but the proof is convoluted". TBC note that convoluted refers to the proof associted with the statement or truth, not the statement or truth itself. Do NOT say "a convoluted truth", that is quite different to a truth with a convoluted proof. {A 'convoluted truth' is what a politician says :) }

To make a coinage, "convoluted" is an Important Associated Single Word (IASW) for the single-word-request at hand. When discussing the issue your question is about, it would be likely you'd use "convoluted" in phrases/sentences - although convoluted is not, per se, a "word which means being definitely true, although not obvious or simple to demonstrate to be true."

Again quite simply there is no sich single word.