There are two symbols I do not recognise below the stretto in this excerpt:
What are these symbols? And also, form what I read the stretto here should be interpreted as an accelerando, am I correct?
This is an Turn, an ornament consisting of four notes. The double-sharp symbol indicates that the lower note to be performed is a g double-sharp rather than a natural g, so the sequnce to be performed is b, a sharp, g double-sharp, a sharp.
I wouldn't interpret stretto as accelerando. My guess is that Chopin put it there to prevent the other natural interpretation of the passage, which is to slow down, from happening. So, instead of doing an accelerando, the passage under the dashes should just be generally faster/flowing, probably slowing down during the last hairpin (>). You'll need to experiment to find a natural way to do it.