Kick harness connects the HAS to the arcade PCB, allowing for buttons 4,5,6 to forward to the DB15 port (aka a complete supergun controller pinout).
Failure to do this will also fail to provide buttons 4,5,6 to the DB15-to-Saturn adapter.
Not exactly, since the HAS can output up to 6 buttons via the JAMMA edge (it's designed so that there is no conflict with the 6th button, don't worry

), because there are games that don't make use of the kick harnesses (Atomiswave, MVS, multi-game PCBs and so on).
Just remember that you need a kick harness for the CPS1/2/3/ST-V and other systems that take buttons 4-6 via auxiliary connector instead of JAMMA, that's it.
About the HD-15:
Can’t you treat it as a direct connection to the OSSC without the need of SCART connectors?
You can, that's how I have my HAS connected with the OSSC, so far all my games have worked just fine. Now with the LPF on the newest HAS you can't even tell if the video is connected via the Scart (for which the OSSC can apply the LPF), or the HD-15.
I've just jumped on the recently re-opened pre-order, very excited!
I wondered if someone could clear something up for me. What would be the use case for the "8pin mini DIN to HD15 adapter PCB"? The description seems almost identical to the "HD15 adapter PCB" above it. Does it work in conjunction with this item? Looking at the pictures it looks like it converts the added VGA port back to a 8pin DIN, what would the advantage of this over the one on the base board? I've likely missed something very simple, apologies in advance!
Different form factor, the same function - both adapters simply output RGBS via the HD-15 connector. The reason there are two adapters is that:
1. The old HAS versions didn't use "modules", so the "8pin mini DIN to HD-15 PCB" is a workaround for owners of the old HAS.
2. Some people already own an 8pin mini DIN to 8pin mini DIN cable, so instead of adding a "HD-15 adapter PCB" to the HAS and using a VGA cable, they can use their 8pin mini DIN cable with that small "8pin mini DIN to HD-15 adapter PCB".
3. The "HD-15 adapter PCB" makes the HAS ~16.5mm taller, some people don't want that.
Also, does the HD15 adapter PCB fit underneath the plexiglass case of the HAS, or does this need to be removed to use it?
All modules/adapters fit underneath the plexiglass case of the HAS, they are supplied with extra spacers and bolts.