As far as i know the external capacitor on RSTn may limit the C2 speed

GET RID OF IT

in the Cygnal days some .... 'solved' noise by putting a cap on !RST and that morphed into an urban legend that the cap was beneficiial BULL....

• What are/is the downside(s) to locking pages other than accidental program erasure?
• none,an issue, not a downside, is that to debug/reprogram, you need to erase the flash and download
• How to choose the number of pages that should be locked?
• just do them all

the one thing you can not change with the crossbar is the sequence of devices.

I recall, in the old Cygnal days asking and was told that they tried a :fully universal" crossbar and it took 3 times more space on the die than the processor

what is a ping-pong transmission

I, probably, know under another name

1) do you have a pullup on !RST ?

2) do you have decoupling no more than 5mm from Vdd?

I see one issue

the delay routine is in C and thus the dalay time is  unknown

use a timer or make (maybe the smallest ever) assembler module

I see one issue

the delay routine is in C and thus the dalay time is  unknown