The offspring of Imrazor and Mithrellas were not Half-Elves, or Peredhil, since this term, in its strict application, refers only to the descendants of Earendil and Elwing who were given the choice between mortality and immortality. This choice was accorded to them by Eru (since the power was his only to give) in recognition their great deeds.
Legolas (of the Downs) put together a very helpful
FAQ on this issue which is on the main site (follow the link).
Thus, the descendants of Imrazor and Mithrellas were simply mortals (ie of the race of Men) with Elven blood.
In other words, one is not a Half-Elf with the choice between mortality or immortality simply by virtue of having Elven blood. The choice must specifically be bestowed, as it was in the case of Earendil and his sons (plus those of Elrond's line who themselves chose immortality).