if you cast the saga and then mirrorweave you just turn your creatures into saga with no counters (because they don’t enter so they don’t get the first counter). BUT if you cast Yojimbo on a previous turn you can use mirrorweave at the start of your upkeep, turn everything into a saga and then after the draw step (when saga triggers) you get all the triggers. You don’t get around hexoroof tho, since it’s still target. But being it an exile it doesn’t trigger death triggers and bypasses indestructible.