JC – There are many answers to your questions in the following articles.
From my investigations it appears that the use of the common seal must be witnessed, however there is an allowance for non fraudulent mistakes in some circumstances. If there are no signatures and no completion of the Attestation then the document with the seal is unlikely to be legal.
https://www.netstrata.com.au/the-common-seal-explained-and-wont-be-found-at-sea-world/
Relevant sections in the SSMA 2015 are: sections 272, 273 of
https://www.austlii.edu.au/au/legis/nsw/consol_act/ssma2015242/index.html#s272
In NSW there is an additional requirement from the NSW Govt Registrar General for consent attestation to be confirmed through the completion of Form 23:
https://rgdirections.lpi.nsw.gov.au/strata_schemes/approved_forms/approved_form_23
https://rgdirections.lpi.nsw.gov.au/__data/assets/pdf_file/0019/215902/AF23_Attestation.pdf