so I imagine it this way..
Soundwave was talked into helping StarScream ..
he calculated better stability realizing that StarScream is a better politician
You know he already had all the remaining Jets on his side already
and SoundWave bribed the Constructs - they need his energon production.
People should see Galvatron as the second version of Megatron and not a separate entity..
Megatron never actually died.. he is a machine.. he was updated. .. and given a different shell
you realize his cannon was 100,000,000x the first one..
lol