Aegon VI's invasion seems a bit left-field and a complication too far, but OTOH it does set things up for Dany's invasion (and Dany and her dragons are a known threat in Westeros by the end of ADWD, so she's lost the advantage of surprise anyway). If Aegon VI is going to ally with Dany upon her arrival, he's already got a toehold and conquered several castles already. In essence he's done what she would have to do anyway. So I think that helps.

The biggest problem with that is Dany still seems a ways from returning to Westeros. She's made the decision that she needs to return home, but she's stuck in the middle of the Dothraki sea with an uncooperative dragon and just been picked up by a Dothraki khal hostile to her. So even in the best-case scenario she's going to have to spend at least a few chapters of Book 6 convincing the Dothraki into an alliance (I suspect naming the dragon Drogon will help, as they may assume his spirit lives on in the dragon or something), return to Meereen, smash up the blockade, deal with Victarion (I have a feeling Victarion will arrive at Meereen first, use the horn to take control of the other two dragons, and then have a showdown with Daenerys on Drogon) and then begin moving her people back to Westeros somehow. That puts off Dany's invasion until late in Book 6 at best.

To get the series done in 7, the only thing I can think of is that Dany's invasion will turn out to be more important in the aftermath of the series rather than the series itself. The war with the Others will be mostly fought by the Westerosi (led by the resurrection-as-PWWP Jon? I'm not a fan of that idea but GRRM seems to have set it up almost to the exclusion of all other possibilities), the Others will be defeated by lore gathered by Bran and maybe Samwell, and Dany shows up in the aftermath. Possibly a little disappointing, but again it would be an inversion of reader expectations.

Based on all of that I am probably marginally (like 51-49) thinking the series may expand to eight books after all.