This still doesn't answer the first question
1) please give a reasonably accurate summary of what the mainstream thinks the Holocaust is. Not what you think the 'holohoax' is.
And you do, finally, provide an answer, of sorts, to the second question.
Whether or not you think T4 was part of the 'holohoax' is immaterial. The fact remains that T4 is regarded as a crucial part of the Holocaust because it evolved methods and trained personnel who were involved in the genocide of European Jews.
If you are conceding that T4 happened, then this immediately poses problems for you, since T4 was carried out by gassing.
and similarly
But the mainstream regards the forced deportations of the Jews as an integral part of the Holocaust. And it also regards a range of other phenomena as equally integral parts, alas you seem unable to spell out what those were.
You see, Saggy, pretty much everyone who has learned even a little bit about the Holocaust knows that 'gassing' and 'mass murder of Jews' were overlapping but not identical phenomena. Many Jews were murdered by means other than gas, and non-Jews were murdered with gas.
Baby-tearing, however, was not among the major methods used, and so cannot define the Holocaust as you seem to think it does.