My math might be wrong, but you'd have about a 50% chance to get her in theory, I think? Because, let's assume that you get one blue orb per summon, but no others, which is not the best or worst scenario. Also assuming you're pulling just blues, which gives you the best chance. So with 80 orbs, at 5 orbs per pull, you'd have 16 blue units. With a 6% chance that a blue unit will be 5* (3% focus, 3% other) you will probably get a 5* blue of some kind, especially factoring in the pity percentage. so it's just flipping a coin whether it would be B!lucina or some random other blue. And, of course, thanks to rng you can have awful luck and get nothing, or get lots of 5*s within those 80 orbs. I'm not a math major, so take these calcs at your own risk
I would roll on the brave heroes banner before doing the free summon. That way you can maximize that 5th orb discount that you otherwise most likely would not have gotten the most out of if you had already summoned for a certain color from the guaranteed banner. As for rolling, the next batch of heroes aren't that unique unless their stats make up for it, so I would roll on the Brave Heroes banner, and see if I can get all 4 before it ends.