China's coast is as most of you said, too large to defend...which is somewhat a worry considering the increase use of special forces means much of China's coastline is vulnerable to special forces insertion from aircraft, submarines and small craft.
Also, Taiwan's far shorter Western coastline is much easier to defend...I'm surprised that we have not seen any Chinese mine clearing devices fitted on its amphibious tanks yet (in fact, I've never seen mine clearing chains fitted to any PLA vehicle).
But in war time, I think the best coastal defense for China will not lie with coastal fortifications, but with mobile missile launchers, self propelled artillery, self propelled air defense weapons, and the deployment of large numbers of air/surface search radar stations along the coast.
Theoretically, if the enemy did land, PLA troops in their sheer size, accompanied with entire mechanized divisions that are now quite capably equipped with IFVs, Type 96 tanks, SP Artillery, and tank destroyers armed with missiles, and possibly with Z-9G coverage, will flush any enemy force off the beaches. In any defense, and now with satellite aids, China could effectively locate the enemy's landing ships while they are unloading, deploy long range rockets armed with submunitions (eg, A100 or WS-1/2) that will deal severely against any enemy force landing on the beaches, and these rockets can easily be fired around 80-100 km from the landing area. Same thing can be applied using short range ballistic missiles (and if necessary, tactical nuclear or chemical warheads).
If the enemy does land on a more remote place, or where the enemy has successfully struck every bridge, railway, and high way that would constrain the main PLA units to deploy against the landing force, remember PLA now has rapid deployment units, particularly the elite airborne troops and the helicopter deployed light infantry brigades, can deploy virtually anywhere in a short instance. They can deal severely against any landed force, even if the main bulk of the PLA can't get there.
In addition, the best thing for coastal defense where your coastline is vast, is recruit reserves, militia, armed police, conscripts etc...whatever disciplined source of man power you can get, train them to fight in small units and equip them with shoulder launched SAM, mortars, infantry portable rocket launchers, and more significantly, ATGM, and perhaps sniper rifles (of the .50 cal anti-material type). Say a light coastal reaction 200-500 strong unit to cover every 200 km of coastline (well, coastline that is deemed to be vulnerable to enemy landings). If the enemy does land there, and the main PLA force was diverted somewhere else or would take time to deploy, these small teams can quickly set up defense positions and conduct small scale harrassment warfare. Each unit would train vigourously and familiarize themselves in their assigned coastal areas, and especially trained in limited guerilla warfare, harrassing the enemy at long ranges, etc.