but was told that if it had got damp at all it would have lumps in it.....
Not necessarily. Not hard, set lumps. The powder may have sections of "lumps" but these will simple break up by just pouring it into your bucket. If you get your hand in there first you can usually tell.
When i say damp as well, i don't necessarily mean wet. Everything your saying almost convinces me that this is the case. Regardless, if it was purchased and used pretty much straight away then your first point of call is the store you got it from.
The tiles that have been removed, are they still fixable, or have you had to skip them?