Can I use cement accelerator to decrease setup time of modified thinset
It is obvious that there is a large concern about getting air to your thinset.  The problem that you have is that there is too much variance in the equation to properly answer this.  Porcelain plus Ditra will cause quite a barrier but there is also airflow of the room and temperature.  

The problem isn't the use of the accelerator, it is the effectiveness of it and how it might effect the outcome.  I have used accelerators for thinset before and have had mixed results.  Well since I didn't live in the houses I am not sure about the results but on at least 2 jobs my crew used accelerators and the thinset became brittle to the floor upon drying.  I would personally never use them and for tile that large I would give it 3-4 days.  But I am risk adverse especially when the risk I am taking is mitigated by me doing "nothing" for a couple of days.

I think your concrete accelerator will work - as far as drying faster.  However will it cure faster or cause an issue with the cure.  I think that will be product/chemical dependent and has to do with the ratios you are using.  Someone may have a better answer than mine but they would have to have tested specific products and ratios and give you their experience - not thinking you will get that but who knows.