Any reason you'd actually not use auto brightness? I've never found a reason in the last couple years to not use it. However even when set to max manual, my S9+ screen (also set to basic) doesn't have the same tone as yours - it's definitely closer to the other phones in your pic.
Between the buttons and screen color - you seem to have gotten quite a crappy sample.
There is no point of testing auto-brightness indoors, even if that is what I choose to use eventually, because that comparision leads nowhere. Phones these days try to detect the ambient light and learn the user's preference, so an objective comparision is not possible in that scenario. Also that would put the S9+ in a disadvantage because the S9+ has yet to learn my preferred brightness.
To properly test auto-max brightness I will need a searing light bulb, or will have to take the phones outdoors in a sunny day. I have had this phone for two days, and for now I am fine testing it against other phones on my desk. I will eventually learn how the S9+ behaves in high ambient lighting, but what I can say with confidence is that my S9+ is not as bright as my S8 or X when I try to change the brightness slider manually indoors.
The "tone" of the S9+'s display is fine. Cameras often fail to capture correct white balance, and from what I can see all phones have acceptable whites.
P.S. WRT buttons, I plan to stop at a store to see if it is just my phone. But if it is an accidental defect, I would have expected it to happen to only one of the buttons, not all three.
Samsung phones have limited brightness if not in auto-brightness mode for many generations already.
But in this case the S9+'s display is not even as bright as the S8's. I did a quick search to learn that I am not the only one who observe this.
https://www.gsmarena.com/samsung_galaxy_s9_plus-review-1736p3.php
http://www.displaymate.com/Galaxy_S9_ShootOut_1s.htm (S9)
http://www.displaymate.com/Galaxy_S8_ShootOut_01.htm (S8)
GSM Arena measured 376 nits for the S9+ and 442 nits for the S8+. Displaymate measured 364 nits for the S9 and 405 nits for the S8. Both publications note that the max brightness is substantially higher under high ambient light (auto-brightness), but even there the two phones are merely equal. So I am left wondering where Samsung's claim of "15% brighter display" came from.
All this does not mean that I think the S9+'s display is no better than the S8's, because it is better. Reflection seems lower, contrast is even higher (I did not think that was possible), colors are more accurate, and the screen uniformity is superior as well. And if the loss of brightness is due to more accurate calibration (which is why often OEMs neglect calibration in favor of higher brightness), then I will take that as well.
But at the same time I do see where Samsung is trying to cut cornors. The whites/colors on the S9+'s screen do not look as "filled" as the whites/colors on the S8's, not to mention the X's. While I need a microscope to confirm this, but I strongly suspect that the panel used on the S9+ is not the same quality panel as the one used on the X (which is made by Samsung!). It is disappointing because I would have thought Samsung would equip their flagship devices with the best of the best. (Also, Apple's color calibration is STILL better than Samsung's, but that is another story)