Don't sign any document other than SPA especially those letter/ instruction asking you to give permission or waiver of your right under Law.
i.e.
1) Alteration/ Revision of Approved Building Plan (effect: they may amend the committed facilities in term of the size/ usage/ etc)
2) Notice of awareness on revise of completion period
3) Awareness cum acceptance (effect: those unsold units may be converted into AirBnb and cause safety etc hassle, share facilities with those AirBnb tenant)