Please post your Qs to the LAMMPS mail list and not
to me directly, so others can answer.
1) the fix dt/reset command can output the current timestep
value to the thermodyanmic output (see the fix dt/reset doc page),
so your log file will know what the timestep is at any timestep
2) LAMMPS will get confused about when to compute dump quantities
(like per/atom energy) if you change the dump frequency on the fly -
hence the error. You would be better off in this case to just close
the earlier dump file (via undump) and start a new file.
Steve