Salam Nicolas,
First of all, "lawyer" in MSA is ttranslated as "ٍ مُــحَــام ". The definite form "the lawyer" is written as " الـمـحـامــي ". As for your question, the correct answer is " مُـحَـامِــيَّ ". Basically it is like adding another "ي" to " محامي", that's why the following diacritic " ّ ", called "Shadda", is used to mark doubled letters. The same rule applies to similar words: ... ,قــاض, ساع, مـاض