The answer, broadly is no, handheld GPS units can not be used for IFR RNAV routes (or approaches for that matter). To know if a specific GPS unit is legal for IFR navigation you need to check up on a variety of paperwork, thats covered in this answer. Strictly speaking if you could find a certified unit it would be legal but there are no current units (to my ...
To my understanding of Part-FCL, yes.
While aeroplanes is a broad term used in the FCL, they use it in some cases to include both, SEP and TMG, e.g.
FCL.105.A LAPL(A) —Privileges and conditions (a) The privileges of the holder of an LAPL for aeroplanes are to act as PIC on single-engine piston aeroplanes-land or TMG with a maximum certificated take-off ...
I'll answer this from a USA perspective.
If you hold a multi-engine rating and/or an instrument rating, they DO NOT expire. Currency determines if they are useable.
If you are getting a new rating (type rating) when that is issued it revalidates the ME and the Instrument rating. You are then current with both and the new type rating. The ME and Instrument ...
It is possible (at least in 2019). The mentioned in the previous answer FCL.610 (IR - Pre-requisites) (see Part-FCL) clearly states that for IR you need NVFR only if you are not going to fly at night:
Applicants for an IR shall:
(a) hold: (1) at least a PPL in the appropriate aircraft category,
(i) the privileges to fly at night in ...
You will most likely need a Supplemental Type Certificate from EASA
According to their FAQ, emphasis mine:
My aircraft has been modified in the USA by Form 337 action. Can EASA accept this?
EASA accepts alterations on non-critical components that are substantiated via Form 337, as detailed in the EASA-FAA Technical Implementation ...