MK152RS - Spinning wheel schematic question

Thanks for providing the schematics for those starter kits- it’s very much appreciated.

I am trying to understand how MK152RS works. Two out of the three sections of the diagram make sense. The part that I do not understand is the clock generator at the top. Here’s what I see:
The momentary switch charges C3 which then serves as “power source/time out” for the circuit. There are two inverters with what looks like a feedback line to the input. There’s another inverter which eventually drives the clock. When C3 is charged there’s going to be current through R4 and R1 towards the first inverter.
If the feedback line was connected to the third inverter I could see how this could be used for oscillation, but with an even number of inverters this looks more like a buffer.

However, since C3 is always going to be positive, how can the input to the inverters ever oscillate ? In other words how can the CLK line ever change ?
What is that R3/D1 line for ?

Sorry, explaining how circuits work is beyond the goal of this forum.
Moderators have limited time and they need to focus on helping people with assembly problems and problems during use.
Maybe a forum user can help?