Thanks for all the replies and comments.
I have great respect for @cathy63's knowledge and have seen at least one professional article somewhere supporting her description in post #22.
I'm not concerned with notions of fairness (post #24). At the moment I'm concerned with understanding the rules in order to help implement the best strategy for the situation I'm familiar with.
The takeaways I have at the moment, assuming post #22 is accurate:
1. While the rule is written simply and is therefore just a rule, the effects in its implementation vary conceptually: the $8M More Aggressive Gifter (MAG) is effectively taxed on their "leftover estate" of $4M and not at all on their earlier gifting whereas the $2M Less Aggressive Gifter (LAG) is effectively taxed on their "total starting estate" of $6M.
2. The LAG really has very little incentive to take advantage of the currently higher exclusion amount. They get taxed on $6M whether they gift or not (well, except any gains on their earlier gifting does end up in the recipient's estate, so I guess there is that), and gifting earlier just may raise concerns in their mind of keeping enough for themselves.
3. The MAG benefits, but not as much as it may first appear. For giving away $8M, they only grow their BEA by $3M (because they'd get the $5M BEA anyway if they did nothing).
@cathy63, do you know if these new rule modifications are embodied in the current estate tax instructions somewhere? And if so, specifically where? I'm going to go look, but I'm not very familiar with the document and it's easy to overlook or miss things of this nature.
Right. The only way you can benefit from the current temporary higher exclusion amount (without actually dying) is to give away more than the exclusion amount will be when you die. If you can't do that, then your estate will end up in the same position it would have been in if there had never been a temporary increase.
As for estate tax instructions, I don't think the form and instructions will be updated until 2026. Until then, the only thing we have would be the examples in the tax code: https://www.law.cornell.edu/cfr/text/26/20.2010-1