Genghis, do you have a reference for; "Differences training is valid for 2 years and If you haven't flown a VP prop in 2 years you need to refresh that BUT it does not legally require flight - a briefing and signature are fine."
Difference training is not a rating and does not require renewal/revalidation, it is simply an experience requirement although must first be delivered by a suitable instructor. Those who had the relevant differences experience before the requirement was introduced are not required to do further training but having said that I await your reference.
JT8D-17 is not required to undertake any formal training on a Europa above that of common sense. Of course a SEP (land), NPPL or LAPL must be valid. On a different matter it is advisable to check with the insurer because they may stipulate that each pilot undertakes formal training from a suitably qualified instructor(FI/CRI).