Scholar makes no mistakes on this matter
Please don't refer to yourself in the third person, its smacks of arrogance.
The necessity of a sign would indicate that the presence could escape their notice and if it required special insight and was over a long period of time then the presence must logically be invisible. So, in short, I have provided three basic facts that prove the invisibility of the presence.
OK, lets look lets have a closer look at your proofs.
1. Request for a Sign
This request of the disciples does in no way proof they were thinking of an invisible presence. You yourself said we don't know what the disciples were actually thinking, so how can you offer this as proof? The scripture in Act 1 v 6 gives us a very good indication as to whether the disciples were thinking of a visible or invisible presence. After Jesus' resurrection and appearance to them visibly, they asked him "Lord, are you restoring the kingdom of Israel at this time?"
This shows that the disciples clearly had a VISIBLE presence in mind when they asked the question at Matt 24 v 3.
2. Presence over a long period of time shown by comparison with the Days of Noah
Again wrong. Jesus didn't compare the parousia with the days before the flood, implying a protracted length of time. The parousia of Jesus is compared with the surprising arrival of the flood itself. A sudden, shocking event. People are in the midst of their daily routines when it takes them by surprise.
The NASV says at Matt 24 v 37-39
"For the coming of the Son of Man will be just like the days of Noah. "For as in those days before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noah entered the ark, and they did not understand until the flood came and took them all away; so will the coming of the Son of Man be."
This scripture shows the normalcy of life before the sudden, cataclysmic event of the flood. This is what is being compared, the normalcy of life before the sudden, cataclysmic arrival of Jesus will be the same. That is why Jesus warns us to keep awake and be on the watch, precisely because of the sudden unexpectedness of his arrival.
This is borne out by Jesus other comparisons like 2 men in a field, one taken along, the other left behind. 2 women grinding, one taken along, the other left. A master arriving unexpectedly.
3. Presence required discernment by the disciples.
No. The Parousia is consistantly rendered in scripture as a sudden obvious event to every eye. It is compared to lightening being visible from east to west. It is compared to a thief in the night, unexpected. It is heralded by celestial phenomena and trumpets. It wil not need discerning but it will need watching for!
You ask an interesting question here
Why did the disciples use parousia rather than coming.? If 'coming' was meant then Why was that word not used for it is there in Greek? These are questions that Reason demands of an explanation. So your argument is nonsensical.
So before deciding that my answer is nonsensical, how about hearing it first?
It is generally accepted by scholars that the primary meaning of parousia is presence. But it also has secondary meanings and a technical meaning. The secondary we have already mentioned as being advent, arrival, The technical meaning is when the context refers to the arrival of a king. In this context the word parousia is translated as coming.
Prof Adolf Deissmann 1908 "Light from the East" says of parousia
"Yet another of the central ideas of the oldest Christian worshep receives light from the new texts, viz parousia, advent, coming, a word expressive of the most ardent hopes of St Paul. We now may say that the best interpretation of the primitive christian hope of the parousia is the old Advent text "Behold, thy King cometh unto thee" Matt 21 v 5. From the Ptolemaic period down into the 2nd cent AD we are able to trace the word in the east as a technical expression for the arrival or the visit of the king or the emperor"
The context of Matt 24 is definately concerning the arrival of a king, Jesus Christ. Therefore the correct translation of parousia from the greek at Matt 24 v 3, 37, 39 is coming, not presence. Also note that the Society claim to use the translation "presence" because they say the context dictates it. (see Kingdom Interlinear Translation). Unfortunately they do not give any examples of why they say this and have just let the argument rest. They have again left the reader to accept what they say without proof.
Not a very good way for "celebrated WT scholars" to act, wouldn't you agree?