I don't have answers to your questions. But I can give you some general
information about sea level vs vacuum engines and about engine counts.
The difference between sea level engines and vacuum engines is mainly
the length of the nozzle. Longer nozzle make more efficient engines but
can make the engine unstable in an atmosphere. That means that sea level
engines will work fine in a vacuum but will be less efficient than
vacuum engines. On the other hand, vacuum engines will tend to tear
apart if used in an atmosphere.
A higher engine count (for engines with similar power) will make you
accelerate faster, which will save you on gravity loss. But when you
have reached orbit you basically no longer suffer gravity loss. If you
haven't yet reached orbital speed but you are close you will have some
gravity loss, but it will be small. So once near orbital speed extra
engines tend to be not very useful extra weight. Therefore the last
stages of a multistage rocket will usually have a lower fraction of its
mass in engines. Once you are in orbit to go higher up or to escape, it
is more efficient to have longer burn times with less engines.
So, a "traditional" rocket that you want to use to send stuff very high
or escape Earth's gravity, usually has a smaller fraction of its mass
for engines in upper stages. On the other hand if you want to optimise
the rocket for LEO, your engines are used before reaching orbit while
you can suffer gravity loss, and you might want to have more engines to
avoid gravity loss.
So why would SpaceX put extra engines on Starship which they want to use
to go to Mars? Well Starship is not a traditional rocket. For trips to
Mars, they want to refuel it in LEO. So when Starship is used to launch
satellites in LEO, you want more power to avoid gravity loss. And when
it is used to go to Mars, well you are really using it to go to LEO
first and you are willing to use extra mass to avoid gravity loss. Then
you refill, at that point, the extra engines are not very useful to goto
Mars but you still have them because you needed them to avoid gravity
loss at launch and they will be useful to avoid gravity loss when you
will launch from Mars.
Alain Fournier