1) Not with the encore reissues, but I did have with earlier ones.
Also my original G1's had been transformed so often that the arms just flapped around and wouldn't stay up... I think its just frequent transformation, and yours is just a manufacters flaw
2) There were diaclone knock offs, but I think its super unlikely you've bought a knock off
3) Yes, buy the uber cheap Encore reissues off ebay, and leave this one for display in vehicle mode, where you won't notice floppy arms. For 40-50 you'l have toy joy again, plus can display them in both modes 
Don't pay extra for the TFC reissue in ANIME colors, its really just a luxury for hardcore collectors.
On the lookout for MISB Headmaster Highbrow, Takara or Hasbro. I'm sure I could make you a sweet deal!