Yes they did use Zyclon B for fumigation purposes. The chambers they used to do that in still survive in Auschwitz-Birkenau. But they are quite different from the killing chambers used in Krema's II - V. For one thing chambers used to delouse clothing were very small, and the concentration of Zyclon B used was very high, so high that the walls of the delousing chambers are stained blue. The gas chambers in the Krema's however were very large, far too large to effectively fumigate clothing. Also the levels of Zyclon B are much much lower (though still detectable in the walls even today).
Some deniers have proposed that the gas was used to disinfect these rooms as morgues. But that is utterly absurd, as Zyclon B has zero effect on bacteria or viruses. Insects are also highly resistant to it, whereas mammals are very vulnerable to it. This is why you have the concentration differences in the two chambers.
Details about the Krema's of Auschwitz
http://www.nizkor.org/hweb/camps/auschwitz/crematoria/
Also I should point out that Auschwitz was a later camp, and not purpose built entirely for extermination. Even Auschwitz-Birkenau was not a fully dedicated killing center, but part concentration camp (and part extermination camp), along with having a 'medical' center for performing experiments. There were also many other satellite camps (50 something), which were slave labor camps.
The earlier Aktion Reinhard camps however were pure purpose built killing centers consisting primarily of Belzec, Sobibor, and Treblinka. Combined they killed more people than Auschwitz (depending on estimates, Treblinka alone comes close to Auschwitz-Birkenau).
http://www.nizkor.org/faqs/reinhard/index.html
As for witnesses, yes witnesses can be unreliable, and yes you have to be careful with what you accept as testimony. But just because witnesses are unreliable, it does not automatically invalidate everything they say (particularly if dealing with large numbers of witnesses saying the same things). It also does not mean that the witnesses are being dishonest if there are holes or errors in their memories. Memory is a rather volatile thing, neither perimant, nor especially accurate. Memory is encoded with emotions, and is frequently incomplete. Also extreme stress can distort memory, or block memory from forming (or also make you never forget no matter how much you want to).
Anyhow, if you are serious about examining the holocaust and your own views on it. the nizkor site has everything you will need. I suggest starting with the 66 questions and answers section first, then moving on to techniques of holocaust denial.
http://www.nizkor.org/features/qar/
http://www.nizkor.org/features/techniques-of-denial/
At the surface, a lot of holocaust denial material seems valid and logical, but do any digging and you will find their claims fall to pieces very rapidly, and that their science is not science at all.
Ok one last comment in general. One thing that always bugs me about people and the holocaust, is the assumption that 'we' could never do such a thing, and that everyone involved were inhuman monsters. Both are false. Any group of people is capable of genocide (and have done such), and most of the people involved in committing the holocaust were average every day people. The commandant of Auschwitz for example, Hoess, was an animal lover, a family man, and the like. Some later turned into monsters, due to the environment, and of course there were other more corrupt and psychopathic individuals (which these places would attract).
But the simple fact of the matter is, this could happen anywhere, and be perpetrated by anyone, be they African, Asian, European, American, Canadian or what have you. It just takes the right circumstances.