The rules for contracts prior to Aug 16 and between Aug 16-Dec 31 essentially doesn't matter to potential lucid owners, because the Lucid is made in America, and that is the only difference between before Aug 16 and in between Aug 16-Dec 31 (all qualified vehicles must be made in America.)
If you have the option to fully sign a purchase agreement and made the full payment for the vehicle before Jan 1, but actually receive the vehicle after, you are very likely to receive the tax credit.
The IRS technically issued an example guidance of what qualifies as a written binding agreement, but not limited to the example of at least 5%. Specifically, IRS issued guidance that a written binding contract is "enforceable under State law and does not limit damages to a specified amount." Although I'd try to ask Lucid if I could increase my deposit to 5% just in case, because we technically have until Dec 31.