Scratch instgated the whole over throw of trizza because the grub meteor would be coming soon and he didn't want there to be two fuchsai bloods.

It wouldn’t matter whether Trizza was alive or not because Condy could easily take her out before Feferi arrived just as well. Also we don’t know how far in the past this is happening? So it may not even align chronologically.

In fact, he instigated Trizza’s ascent to power as implied by Dammek’s scrawlings. Whether he’s also plotted her downfall, that’s yet to be seen.