Saturn does not even twinkle lol.
lol actually Saturn does 'twinkle' or scintillate to give it the technical term. Unlike the stars, the planets are not true point sources of light and so the twinkling effect is not as obvious as it is for the stars. The twinkling effect as I'm sure you know is entirely caused by the atmosphere and since light from the planets passes through the atmosphere on its way to the observers eye just as it does for the stars, then it follows that the planets are affected as well. The behaviour of light as it passes through the atmosphere is different between a point source and a disk and those differences are the reason why 'twinkling' is far less apparent for planetary disks than it is for point sources.
At the moment the planets Jupiter and Saturn which are easily visible to the S (northern hemisphere) or N (southern hemisphere) are well to the south of the celestial equator. So from mid northern latitudes (40-50 degrees say) then there may be a slight twinkle visible because they only rise a few degrees above the horizon. For latitudes near the equator and especially to the south where the planets will be higher up in the sky the twinkling effect will be negligible and best suited for imaging.
Yes Venus is brightest when at it is visible as a thin crescent phase and that is for two reasons. Firstly the disk size is greatest when Venus is at is closest so it follows that it would be brighter compared to when it is on the far side of its orbit compared to the Earth. It is then also very small and much further away. As you will know if you have watched Venus through a telescope, just prior to inferior conjunction (Venus between the Sun and Earth) the disk size can exceed a minute of arc where as close to superior conjunction (Sun between Venus and the Earth) it shrinks to slightly less that 10 seconds of arc. If you take the total area on the sky that Venus covers when a thin crescent, it is still greater than it is when a very small (and much fainter) disk. Just because most of the disk is not illuminated when a crescent that is irrelevant as far as how bright it looks from Earth.
Same reason surveyors assume it.
Over the sort of distance scales that surveyors are typically working at of course you can assume a flat baseline. A bit different to working with the distances involved in interplanetary space!
The assumption that Venus has a magic atmosphere with special reflective properties
Nothing magic about an atmosphere that has an albedo of 0.75 compared to the Earth at just 0,39. Sulphuric acid, which is a major component of the Venetian atmosphere is highly reflective. Venus is a classic example of a runaway greenhouse effect which is why the surface is hotter than that of Mercury.
All mainstream astronomical observations based on the globe model are a fiction.
Is that right? My experience and yours are obviously very different but whatever you say my friend.
In short, Somerled is very good at being dismissive about a subject that they evidently don't actually know much about.