I cannot comment on BDW's English either; I believe it's good enough to get her thoughts across verbally and in written. But this is hardly the point.
It is true that Capacity is one of the pillars that make a contract stand. For example, if a contracting party is a minor, mentally disabled or intoxicated etc., the contract should be void. However, illiteracy alone is not a qualifier of "incapacity". An illiterate person still has the capability to seek help, understand and agree to the terms of a contract, therefore can be legally competent. In real life, we all know that one can hardly use "poor English" as an excuse to escape his obligations.
You raised an interesting point here: "right to know" was violated, leading to breach of "informed consent". Minto's act of removing the floor plan pages (on which BDW initialled) without explicit notification is tricky and disgraceful - that's a mild way I can put it, but I am not sure to what extend it could be accused of forgery. Note that the contract was not binding yet by the time it came back from the Minto HQ. In normal practices, the buyer shall be given 5 business days to have his/her lawyer review the contract after both parties have signed, and if there is no objection raised after the period, the contract is in effect. BDW's lawyer should have reviewed the contract in which the floor plans were missing, and it would be a huge mistake if he had not explained to BDW what this implied.
These are just my humbled opinion as an amateur bystander.