Try holding Volume Up, Home and Power Button like described in the following guide: https://support.bell.ca/Mobility/Smartphones_and_mobile_internet/Samsung-Galaxy-Tab-S2.how_to_perform_a_hard_reset_when_my_samsung
Since there is only one micro port for charging the tablet and one headphone port, I can't imagine where you would connect an adapter. Therefore rather no.
Used units are marked as "Gebraucht" or "Neuwertig + Geprüft", see: https://www.digitec.ch/de/s1/secondhand/sector/digital-1 The price difference in the colours is probably due to supply and demand.
No, there is no difference. Maybe the colour gold is sold less, it has a bigger stock, or a cheaper purchase price and that's why it's cheaper. Well, gold looks different to me. If you like the colour, then buy it at the cheaper price.
Spen is not possible. There is also no S2 version with Spen. There is only a Spen version of the 2015 Tab A. Possibly the Huawei mediapad premium version, which has an active stylus, but is not supposed to get Android 6.
Unfortunately, we do not have any precise information on this. However, we assume that this would not work without a corresponding driver. However, we would recommend that you contact the manufacturer's support so that you can obtain more precise information.
The only difference I found is a new CPU in the 2016 models.
2015: Exynos 5433, quad-core 1.9 GHz & quad-core 1.3 GHz with a Mali-T760 MP6 GPU.
2016: Qualcomm MSM8976 Snapdragon 652 - quad-core 1.8 GHz Cortex-A72 & quad-core 1.4 GHz Cortex-A53 - with an Adreno 510 GPU.
In other words, a new processor from Qualcomm instead of Samsung's own Exynos.