Dont do it in a DA42 - you will be restricted to the 42, single lever ops etc.
A fairly wild exaggeration. If you do the initial MEP course on a DA42 (EFIS and FADEC), you will need differences training before flying a conventional twin. However, as differences training is required to transfer for the first time between any two MEP types (e.g. BE76 to PA34), this is hardly a significant problem. There is no such thing as a 'glass sign off'.
The rules haven't changed, it's just that you clearly do not understand them.
If the object is simply to gain a JAA MEIR prior to the adoption of the EASA implementing rules, with no immediate intention of using the MEP privileges, then the DA42 is probably the best option. It should be both cheaper and easier to pass the skill test in that aeroplane than in a conventional twin, especially if you go far enough south. The fact that the IR is done outside the UK is entirely irrelevant and will not be apparent from the rating entered in the licence.
The revalidation and renewal requirements for a MEP class rating are the same - a Licensing Proficiency Check. The MEP class rating can be revalidated/renewed at the same time as the IR only if the LPC is conducted in an aeroplane (i.e. not in an FNPT). However, if you have no interest in twins per se, why bother to revalidate? You can maintain the IR on an SEP and simply renew the ME privileges if and when you want to use them.