It is very simple really and already put here, but to summarise:
If you are a PPL holder flying with an FI, then you can only log P/UT NOT P1/S or PICUS.
Only a successful flight test is able to be counted as P1/S.
Bookworms post simply shows that you can only have a co-pilot on a multi-crew a/c. SEP's etc. are classified as single crew. So there is only one pilot needed. The only difference is when an FI is onboard and two people can log the flight, but only one can be captain ie P1.
However, as a caveat if I'm doing a checkout with someone and I don't have to touch the controls or utter any "advice", then I tell them to log it as P1 and I won't log it at all. An hour here or there makes no difference to my log book and as long as I get paid I'm happy.