Hi gonzo,
Please find below three excellent articles explaining Algorand intents. If you have time, please read them. They are very well written and it’s no rocket science.
Particularly, the last link will answer your questions.
First, the 2.5bln Algos (originally 3bln) are NOT participating in the consensus protocol.
Second, an amount of 500mln stake keys held by the Foundation DOES participate in the consensus protocol. This is (taking again block 972766 as an example) 0.5/1.5 is about 33% of the total. Algorand states that this will never be more than 49%.
Third, the way Algorand moves forward toward total decentralization is given in the first article.
Thanks for the good questions.