Regarding the multi-crew pilot (ATPL) wanting to do some single pilot stuff on the weekend - no, another check will not be required, as long as one has been done at some point (ever, not necessarily your last check). See 61.675
Recent experience requirements, however, include 1hr of flight time and an approach every 6 months. If that expires then I GUESS you need a check done or an hour of ICUS (or the new equivalent). 61.690
The single engine after a multi-IFR check seems to be a non-issue also.
61.695 allows you to fly an aircraft of a particular category. The categories are fixed-wing, rotary, etc., NOT single-engine/multi-engine.
61.695 (2) then prevents multi-engine fixed wing ops unless you've done the check multi-engine.
Its the strict liability stuff with logbooks etc that I'd be more worried about.. oh, and do I REALLY have to write VH- in the registration box of every single line in the logbook?