No. The Display Dock has two USB-C connecting cables to the host. Only by plugging in both connecting cables to the host, the full functionality of the all ports can be used. As the 12″ MacBook has only a single USB-C port, it makes full use of the Display Dock impossible.

No. There are a few limitations as to which combinations of ports can be used at the same time. Basically, it is the HDMI port plus another port from Mini-DisplayPort, DisplayPort or VGA or the DVI port plus these ports. HDMI + DVI at the same time will not work as well as the combinations among Mini-DisplayPort, DisplayPort and VGA.

While advertised as being capable of displaying at UHD (4K) with a maximum refresh rate of 60 Hz, some monitors can do this only via their (Mini-)DisplayPort while their HDMI port can do 4K @ 30 Hz only. Check the specs of the monitor carefully to find out if your monitor really can do the 60 Hz via HDMI.

In order to use the Mini-DP port as an HDMI output and connect 2x HDMI monitors or TVs, you must use an active Mini-DP to HDMI adapter or cable (must be “active”!).
An example of an active adapter is P/N 11892, LMP Mini-DisplayPort to HDMI adapter.

It is normal that any USB-C hub or dock becomes hot when attached to the laptop. The aluminum housing is meant to dissipate the heat from the inside to the environment. There is a number of chipsets working on the inside which are producing some heat since they are constantly in a stand-by position to react quickly should any port be used. The surface temperature is well within the allowed limits and reaches about 40° C in stand-by mode, 43° C under normal use and a max. of 48° C when all ports are in full use, CPU load at 100% and PD attached. The heat does neither pose a danger to the laptop or any attached device, nor does it influence the USB-C hub’s/dock’s performance or even shorten the life-span.
Furthermore, the surface appears to be hotter than it actually is since it is touched with the very senstive fingertips. We suggest to remove the USB-C hub/dock when not needed in order not to emitt unnecessary heat to the environment and not to use any energy.

Basically, you can use only the iMac (27-inch, Late 2009) and iMac (27-inch, Mid 2010) with a Mini-DisplayPort connection with the current Mac with Thunderbolt 3 (USB-C) ports. You need a USB-C to Mini-DisplayPort adapter (e.g. 16134/16138, LMP USB-C to Mini-DisplayPort adapter) or a USB-C dock with Mini-DisplayPort port (e.g. 18641/18645/19009, LMP USB-C Travel Dock 4K 9 Port or 17113/17278/18954, LMP USB-C Compact Dock 4K 8 Port or 17116, LMP USB-C Display Dock 4K 10 Port). Then a Mini-DisplayPort to Mini-DisplayPort cable (8305, LMP Mini-DisplayPort to Mini-DisplayPort cable) must be attached to the dock and the iMac.
With iMac (Mid 2011 to Mid 2014), it is possible to use the iMac as monitor with a Thunderbolt connection only. With a newer iMac, a Target Mode with an iMac is not possible anymore.
For further details, see https://support.apple.com/hr-hr/HT204592