I have to confess I haven't read the entire thread, just the first few hundred posts.
However, I do think there are real issues of tolerance here. There seems to be intolerance towards disabled people being inconvenienced (sometimes seriously), there is also intolerance towards mothers who, let's face it, aren't being unreasonable in having children, but are somehow expected to become social pariahs by a few posters. Heaven forbid they should have their lives made easier when they CHOSE to have children.
Fwiw, some have mentioned mothers of twins - well this is usually not a lifestyle choice - you choose to have a baby, not to have 2, and from seeing my friends with twins, it is TEN times harder with twins.
To those who said 'change a baby/toddler on your knee' - there is no way on earth I could have done that with ds1 or dd, both of whom had 'toddler diarrhoea' - ie nappies that were enormously full of acrid, liquid poo, leaving them with horrendous nappy rash, and an absolute necessity for a changing table and a sink. No way could they have either been changed on a knee, or left until later, as one would have entailed a parent being covered in poo, the other would have left them bleeding. Ds2 and ds3 however I often do change on a knee, because they are far FAR easier.
As far as the OP goes, I had a slightly similar situation the other day, but I don't think I did ANYTHING wrong at all. I was in a shopping centre, needed to change the baby. The baby change area was in a weird sort of combo with the disabled toilet - an outer door led into the baby change area, then a further toilet into the disabled toilet. Both doors were lockable, and obviously unless you locked the outside door you wouldn't be preventing a disabled person from using the toilet. So I took ds3 into the baby change area, changed his nappy, but obviously needed to wash my hands, and the sink was in the toilet area. So I pushed the (unlocked) door open, and to my horror someone was in the toilet (I didn't 'see' them, but they SCREAMED at me, along the lines of 'Get the hell of out of here, this is the f*ing DISABLED toilet'. And some more choice insults. Even though I don't think I did anything wrong at all. And didn't even go into the room - she could have said 'sorry, this room is busy'. But didn't.