Scholar makes no mistakes on this matter and niether does the FDS and its 'celebrated' WT scholars. Fine tuning or adjustments in the pursuit of scholarship and the spiritual nourishment of the Lord's people ia all that is necessary.
The notion of invisibility is wholly ensconsed to the Olivet Discourse and the Synoptics. A little common sense and reason also has much merit. No one knows what was in the minds of those few disciples on the mountain. All that we can know for certainty was the question put to Jesus and Jesus' answer to hHis disciples. The context of the narrative and the vocabulary are all part of the understanding of the nature of the Parousia and the Generation. If the Parousia was momentary or the last event as alleged by Leolaia then 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.
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.
1. Request for a Sign
2. Presence over a long period of time shown by comparison with the Days of Noah
3. Presence required discernment by the disciples.