At least the activity is south of El Hierro for now and that is most likely where a collapse would occur. If it does, it should lessen the force of a tsunami to the the other islands, northern EU countries or our East Coast?
Then again, these quakes started on the other side or underneath the island and worked themselves South until the magma vented. There could be a chance of the venting working its way back underneath the island. Imagine the catastrophe if the island split in half and one or both halves collapsed into the ocean!
As for the quakes....my understanding from a geologist in Spain is that there is about 40k Olympic sized swimming pools worth of magma pretty much just sloshing around and through the rift zone. There is no natural chamber above 15km so it's fighting pressure from beneath to empty. Interesting is that the hotspot is located right under the western most side of El Hierro.
All this added up, I think at that depth if the magma ever does find an outlet the resulting quake will be between 5 and 6 in intensity. Will that be enough to trigger a collapse? Certainly the risk is rising the longer this plays out with the volcanic tremor being present for so long.