Hi Gaël,

> 1- Is the 2nd pers. pl., pres. indic. "hefjið" or "hefið" ?

In Modern Icelandic "hefjið", in Old Icelandic/Norse "hefið".

> 2- Is the root vowel in the past subjunctive "æ" or "œ" (as in fara) ?

In early Old Icelandic/Norse it's "œ". (This sound was unrounded in
the later 13th century till it was pronounced the same as "æ"; Modern
Icelandic spelling uses "æ" both for original "æ" and for the
identically sound descended from "œ".)