What if

[Mercen-X]

Due to his increased mass and non mako-infused constitution he probably would have made a big splat.

I'm imagining everyone would have ended up in jail in Shinra HQ sooner or later anyways.

You've stoled me mind, freak!

Assuming he survived... waking up in the church, Barret would apologize about the flowers and promptly attempt to leave noting the look of shear terror on Aerith's face at encountering a giant black man with a gun for a prosthetic. He would then bump into Reno on the way out who would react much the same way as Aerith, falling back to a "safe" distance and ordering his men to apprehend the "big golem". As Jiro said, there would be a shootout and Barret would win (presuming the reason his presence at the pillar didn't help because he was holding back to prevent damaging the pillar). Reno would fall back to summon reinforcements and Aerith would suggest Barret take the back way out to avoid attention. Barret would tell Aerith to stay in the church thinking the Turks are after him but she would follow after him anyway.

Aerith would follow Barret all the way through to Sector 7 where they would encounter the ShinRa troops moving in to attack the pillar. Barret would ask Aerith to grab Marlene and keep her safe as he ran to the pillar seeing AVALANCHE already making their way up.

Of course, in truth, if we only switched the roles and not the perspective, then we would see none of this only hearing about it when Cloud returned after helping Tifa interrogate the Don.

If Cloud had been up on the cat walk with Tifa, he would have succumbed to her complaints about having to find Barret. Curious how Shinra knew about their attack on the second reactor, Cloud would interrogate Biggs, Jessie, and Wedge all of whom would deny any ties and Tifa would vouch for them. A member of AVALANCHE catches a spy trying to run to Sector 6. The squeeze him for the Don and Tifa and Cloud head to Wall Market to have a talk with him. Tifa not being a silly as Aerith or recognizably weak goes into the Don's alone while Cloud waits outside. Tifa returns a few minutes later with what she got out of the don.

They return to Sector 6 to see Aerith entering Sector 7. Upon their return, they see Barret making his way up the pillar with Biggs, Jessie, and Wedge and run to join him. Tifa stops to consider Marlene, but a shopkeep nearby informs her that she's safe. At the top of the pillar with odds 6:1, Reno cannot engage the pillar detonation and is forced to retreat.