The 1837 edition dropped the and preceding the second after in verse 6. Yet there is no real reason why the original and should not be there. The RLDS text restored the and in the 1908 edition since the conjunction was never crossed out in the printer’s manuscript. Perhaps placing dashes around the second after-clause might help the reader process this passage:
The demonstrative this near the beginning of verse 6 does not refer to the miracles Jesus will perform (referred to in the following phrase) but instead to what he will suffer (mentioned previously in verse 5). Thus the original and is necessary for maintaining the original distinction between what Christ will suffer and the miracles he will perform. Since Joseph Smith did not mark this change in the printer’s manuscript, it is quite possible that the loss of the and was simply a typo in the 1837 edition.
Summary: Restore the original and before the second after in Mosiah 15:6 (“and after all this and after working many mighty miracles”); the demonstrative this refers to the preceding language rather than the following clause.