I have never replaced a head assembly, but I've recovered data from
opened hard drives in very filthy environments and had remarkable
success (for example, when one head crashed). Your platters are
certainly not damaged at this point just because you opened the drive.
You can't consider your rebuilt drive to be as reliable as a new one,
but it will most certainly last long enough to get the data off.

I am not sure alignment is an issue either; the sectoring on the
platter should handle this.

To keep the heads apart, just wedge something in betwen the arms;
don't put anything in contact with the heads.