The 1874 RLDS edition inserted the modal verb may here in Mormon 7:9. This reading has been followed throughout the subsequent RLDS text. This intrusive may in the first if-clause is undoubtedly an accident and was prompted, it would appear, by the may in the immediately preceding “ye may believe that”. The following if- clause doesn’t have may (“and if ye believe this”), which shows that “ye believe” is correct for both if- clauses.
Summary: Maintain in Mormon 7:9 the original text without the intrusive may that unintentionally entered the RLDS text in the first if- clause.