Coal fires affecting seismic activity, and also volcanoes
I'm assuming you mean underground fires within coal seams whether they are caused naturally or through human neglect or error.

Regarding the possibility of them "triggering volcanic eruptions by affecting the flow of magma or by some other means", the answer is no.

Underground coal seam fires are mostly smoldering type fires rather than raging fires, due to the lack of oxygen underground. Most are located far away from sources of magma: either laterally or in terms of depth.

Concerning whether coal seam fires can "cause, ... prevent or postpone earthquakes", they cannot prevent or postpone earthquakes. Can they cause a seismic earthquake? Unlikely. 

Coal seam fires create a void underground. If the fire is extensive and the extensive burnt out zone abuts an active geological fault the resultant void may cause a ground stress redistribution which might cause a minor earthquake. The other possibility is that the geological strata overlying the burnt out void is weak and eventually subsides, either slowing or quickly. A quick subsidence can create a minor earth tremor and could cause a hole to puncture the surface of the earth, depending on the depth of the coal seam and the void.