As to the first question, you can execute another Sub by Calling it.  In fact,
it is a good general idea to do exactly that.  Break longish code into
multiple Subs, and Call them.  The other Subs can have input arguments.

It is a good idea to do this, as it makes debugging chunks of code MUCH
easier.  It also makes reading thing easier, as you are dealing with logical
chunks.

Sub DoLots()
Dim strIn As String
Dim oBM As Bookmark
' yadda yadda...some stuff
' giving value to the string
' setting a bookmark
' WHATEVER

  Call DoOtherOne (strIn, oBM)
  Call DoSomethingElse
End Sub

Sub  DoOtherOne(strBlah As String, ThisBookmark As Bookmark)

' do something with strings and bookmarks

End Sub

Sub DoSomethingElse()

' lots of other crap
End Sub

As for question #2, user selected macros.  You could certainly take a user
selected string, and make that a variable, then pass that to the Run method.

Look up the Run method in Help.: