Can I apply earlier than 90 days before arrival?

It is not suggested to do so. Usually, the validity of a single entry type is 3 months (90 days). If you apply 90 days in advance, yours may expire when you use it.

May I know you will only land once in China at Shanghai? If not, you can try to apply for a double entry valid for 6 months, so that you can apply around 90 days in advance.

I have now spoken to the cruise line's preferred visa services agent who said that because of the time constraints involved with an extended cruise itinery they have a special arrangement with the Chinese Immigration service to process applications more than 90 days before the entry date to China.