Sally 6502C CPU to U1MB Adapter

No, It is a regular DIL socket.

OK, in this case would it be worth ordering thinner PCB, so the pins in the sockets are as long as possible?

you can try but standard thickness is perfectly OK

Indeed, if there is strange behavior or instability of the Atari, connecting to the Phi2 buffered signal from the Antic chip is a good idea. In most cases, however, taking signals from the CPU simply works and follows the manufacturer's instructions: https://lotharek.pl/img/cpu.jpg

The documentation you are referring to is outdated and no longer valid. The Buffered Phase 2 Clock (BO2) is the main clock for all Atari chips, including cartridge port extensions. According to the 6502 NMOS chip specification, the O2 (PHI2) signal should have no more than 2 TTL connections and should not be loaded with more than 130pF. On the standard Atari motherboard, the O2 signal is connected to the 74LS08 4-AND chip to provide a buffered signal and directly to the Freddie chip. This is why the U1MB should be connected wherever the BO2 signal is provided. Perhaps in spare time, this board could be updated to reflect the current state of knowledge.