Oh, the mess you've stumbled upon. Basically, you're right. There's a lot of people who say what you heard. And lots of other people who say it differently.
In theory, the "x" between vowels should sound as /ks/ (like the x in the English taxi) and, although it's a fact that many people just say "esacto", that's not the proper way.
In fact taxi being pronounced "tasi" can be seen as vulgar but is totally understandable and used.
About "fascinante", the "sc" is indeed mostly pronounced as "c" by everybody who uses the /z/ sound for the "c" and as "s" by the people who uses the /s/ sound for the "c" ("seseo"), and sometimes the /s/ sound wins even among those who don't do "seseo". The combination /sz/ tends to be seen as too complicated. But, again, the proper way is to do the /s/ sound followed by /z/ - I haven't heard (in the 15 years I've lived in Spain) anyone say it like that other than facetiously, except maybe on the daily news or similar.
Finally, "excepción" is a mix between the first case and the second, it's hard to do the whole /ksz/ sound so most people just do /kz/ (or /ks/ in the "seseo" zones). The full /ksz/ sound would be seen as facetious, but just reducing it to /z/ would be seen as vulgar/uneducated.
And there's another school of using the /j/ sound for the "x" so you might even hear /jz/ or /js/ on "excepción" - especially near Madrid (they are famous for their j) - but the /j/ sound is not used for every "x", you will never hear "taji", for example. On the first link there's also an explanation about how and when "x" should sound as "j".
Strangely, the "taxi" to "tasi" and similar deformations just don't happen in the parts of Latin America I'm familiar with (Chile, Argentina, Perú, Bolivia, Costa Rica) where they do "seseo". The other two cases also apply on their "seseo" versions for these countries too.