Most J tickets come with lounge access in China. Notable exceptions are where an airport doesn't have any sort of lounge, and/or you bought a special type of J product where you only get the "in-air" service but not the "on-the-ground" components, such as lounge access. Such product is offered by several Chinese airlines, usually as an upgrade.

There are no OW carriers in mainland China, so OW status will not get you into anything in Y.
ST rules stipulate that you will need to be travelling internationally to be eligible for lounge access in Y. There is a special provision for MU/MF/CI elite+ members to access lounge on a domestic only Y ticket though.
*A is probably the easiest one - with *G if you are travelling with CA or ZH you can get lounge access in Y, much like if you are flying with them in J. There might be a few exclusions at smaller airports where CA/ZH cannot/chose not to contract the airport-operated lounge for *G.

The quality of lounges varies a great deal, ranging from amazing (e.g. CA's new lounge in TFU and KWE) to downright abysmal (some of the airport-operated ones, especially in small regional airports).