It doesnt matter which one was the main objective, the important point is that a half invaded china was not enough for Japan and a "political victory" was not posible. So Japan total occupation the china was a real objective but they couldnt archieve it in 8 years.
Why 8? Japan started conquering China in 1931, from Manchuria. That's because it wasn't actually "main goal of Japan", it was "it would be nice to have all China, but...". Then different factions add different "but".
And there was a party that believed "what but? nothing but here".
After all they only attacked the isles AFTER the stalemate in China, wich doesnt demostrate that china was more important for japan than the whole pacific,
Do you remember that in 1931-1940 conquering of European colonies in Pacific was unscientific fiction? Yes, Japan had two lines of expansion then - China or Soviet Union. They tried China and succeded. They tried Soviets (from 1935, by the way) and failed, so they don't push that line. Remember Battle of Lake Khasan - it's 1938; do you think Japanese did it to improve their position in China? Or Khalkhyn Gol?
After continental Europe fall and colonies became very vulnerable - then Japan attacked, not opposite way.
the chinese coast wasnt the objective, whole china was.
Chinese wasn't fans of giving all the coast to Japan. They just don't surrender.
Actually that's why Navy doesn't want to be involved (check their directive in 1937) - they understood that would mean fighting in the middle of China, and "why would we?".
Its not necesary to talk about factionalism (tho I admit its very interesting), its not like they were warlords or parties with their own army or that they were fighting separate wars.
But they were. They were exactly that. That's why IJN had it's own tanks (embargo, remember?), and IJA had it's own carriers (embargo, remember?). They were formally allied parties (it was written documents where Navy promise to help Army or vise versa, and they needed Emperor's sanction to achieve it, but they existed), but they were different parties who were fighting separate wars.