Entry Before Date Problem

I leave UK on 9 Jan on a World cruise; enter mainland China only once (Shanghai for 2 days) on 14 &15 March before going on to Hong Kong; no re-entry involved.
I understand that it would be necessary for the visa to be issued from 15 December (possibly 17th to allow a buffer).
I am concerned about applying (sending the paperwork) for it on say 14/15 December due to the potential problem of Christmas post or, worse still, postal or other strikes. Is there any way that the ‘valid for 3 months from the date of issue’ aspect can be dealt with?

Is it true that as long as I enter China before the enter before date I can (theoretically) stay for 30 days from that date?