Electronic – Drilling hole through PCB ground plane

The mounting holes that have the large copper pads around the hole are fabricated that way because it is desired to have that connected circuit be conductive to the mounting screw. In most cases it is the GND of the circuit that connects to the mounting hole pad and there is a desire to have such GND connect into the metal chassis to which the board mounts.

The small via holes in the mounting hole pads are designed to electrically connect the mounting hole pad to the pad on the opposite side of the board. In the case of a multi layer board with an inner plane layer the vias may also be used to connect the mounting hole pad to that inner layer.

In the past it was much less common to see mounting hole pads with these via holes. Instead the mounting hole was built as as a large plated through hole to connect to the opposite side and/or inner layers of the board. However, as in the examples you show, note that the more modern type mounting holes do not have plating in the large diameter screw hole and thus the need for the vias to provide the electrical connection.

Plating removal in screw mounting holes is primarily done for one reason. The sharp threads of a screw in the hole can cause small particles of metal to come off the plated hole. This is particularly true when boards might be removed and re-installed during testing and/or repair. These small particles can come out and appear on the circuit board or float around in the electronics enclosure. In the advent of surface mount components with vary narrow lead spacings these small metal particles from the screw holes can lead to shorts on the circuit leading to either intermittent circuit operation or outright failure.

If you don't want the leads to protrude on the bottom side you essentially want to mount your TH components as SMDs. So I would suggest to use SMD pads, push the leads onto the pads, and solder them there.

For large/heavy components this might give a mechanical reliability problem: a round wire soldered at both sides (or soldered at the bottom only but with a component immediately at the top) can't easily rip the copper from the PCB, but a lead soldered to an SMD pad can!

PCB material is not optimized for conducting heat, so you might want to put lots of via's in the appropriate places to conduct the heat.

I read in your edit that you want to do this for trough-hole screw connector blocks. My gut feeling is that this will seriously impact the mechanical reliability. You could consider using those two-level connectors for screw terminals, which would free room so you could put your heatsink under only a part of your PCB, leaving the area of the screw terminal terminals to be soldered at the bottom.