If Tsitsipas/Tsitsipas win their quarterfinal, their combined ranking will increase to 68 and they will definitely qualify. Then Draper/Norrie will still have 79 because they are qualifying based on their singles rankings, and they can't drop any more than that and would theoretically be the first alternates. Haase, Erler, and Miedler can drop in the rankings if the Greeks make the final in Paris, and the Austrians also if Galloway wins the doubles title at the Surbiton Challenger this week (he is currently in the quarterfinals). If either of those happen following a GRE win tomorrow, then GBR 2 will be safe and the last spot will be between AUT and NED 2.
Of course, probably there will be some withdrawals, especially considering the status of Cilic, Shapovalov (does he even want to play the Olympics on clay?), Daniell, and even Djokovic now which would remove Serbia. So probably in the end all of GRE, GBR 2, NED 2, and AUT will get in, and we will see about BRA, MEX, HUN, and possibly SRB (if no Djokovic) and CRO 2 (if no Cilic).