1. Yes, the SPA has to be signed. Otherwise, it will not be considered as public document, and any transaction involving real property would be void.
2. The rules on notarial practice require parties to be personally present before the notary public. Also, any disposition of real property would require marital consent (because the property is presumed conjugal). Therefore, the spouse is considered a party to the transaction.
3. Yes. Only notarized deeds of sale will be honored by the Registry of Deeds. Without it, you will not be able to register the property in the name of the buyer. Besides, the deadline for paying the capital gains tax and documentary stamp tax on the sale would be reckoned from the date the deed was notarized. And without payment of taxes, the BIR would not issue a "Certificate Authorizing Registration" or CAR. Without the CAR, there is no way that the Registry of Deeds would transfer the title of the property to the buyer. https://www.alburovillanueva.com/land-titles-real-property-registration
4. Same as #2.