Dear Florian,
Q1. So, since April 2017 we have 470 available images in L2A for S2A. Does it means, that we have 1047-470 = 577 images available before April 2017 (in L1C)?
A1. Yes, you are right, but pay attention because some S2 data could be replicated due to the re-processing when a new baseline was released, if you have more than one S2 tile acquired on the same day I would suggest to use the ones with the most recent processing date.
Q2. Why there are no corrected images (S2B_L2A) between July and December 2017? Is the preprocessing worklow considering something like cloud coverage and it eliminates images with 100 % coverage? Or is there also another explanation?
A2. You do not find S2B_L2A between July and December 2017 because the systematic processing of S2B_L1C to BOA started in December 2017 after the commissioning phase.
If you need more information on Sentinel-2 data you can have a look on https://sentinel.esa.int/web/sentinel/home or contact EOSupport@Copernicus.esa.int.
I hope that my answer was clear, but do not hesitate to contact us for any further clarification.
Best Regards,
Fabrizio