[meteoestarreja]

1 - For test purposes, can we use a polyester 100nf capacitor rated at 250V with a 5% tolerance (instead of the ceramic 50v model). Or does it smell like... burned stuff?

The biggest difference (I believe) is that polyester ones doesn't do a good job with high frequency signal compared to ceramic one.
About the voltage, you have a 50V rated capacitor, until 50V it does work as supposed no matter if you work with 10 or 40V, higher than 50, it became "out of range" and it can even "explode" (we did that alot just for fun in classes)

2 - Also for test purposes, since they are more readily available, I suppose there is no problem temporarily using carbon - 5% tolerance - resistors?

I believe that won't be a problem, but maybe someone around here already tested ...