ok for the 1st one
find
<p><em>(.*)</em></p>
replace <p>\1</p>
for the 2nd one, use 2 passes - first remove the not-needed inner bits
which start with a close em tag followed by an open em tag:
find </em>(.*)<em>
replace \1
then use a 2nd pass to change em to strong
the trick is to use several simple expressions not one very complicated one, and review results after each stage.
make a backup before risking a replace all
if you have the patience, step through using find replace to do single operations and then move on to the next candidate, that way you can skip past any o you want to leave unchanged
NB I do all this using sigil - syntax may be different for other tools