Heir apparent here is a fixed phrase. It is a legal term and descends from the two or three centuries after the Norman Conquest when laws were written and legal proceedings conducted in a variety of French, which ordinarily postposes its adjectives.

Heir apparent does not mean "seems/seemed to be the heir" but "is/was the heir". The heir apparent is the person who must inherit a title if he or she lives long enough to do so. The term contrasts with heir presumptive, who is a person who currently stands to inherit but whose right of inheritance may be broken by some external event. For instance, if the Earl of Greystoke has no children, his younger brother is heir presumptive; but if the Earl then marries and has a son, the son becomes heir apparent to the title, and the brother has no right to succeed unless the son dies before him and leaves no son of his own.

In the Einstein use, of course, no actual inheritance is involved; the author employs the term very loosely (and obviously without thought) to mean "Newton's successor as the single physicist regarded as unquestionably pre-eminent in the field."

"Scientist of genius" may be used interchangeably with "scientific genius"; they're both basically honorifics without much specific content.