You’re mostly right. Fewer, bigger balloons means less balloon material to hold the same amount of helium.

However, Valentyna barely has any midair control at all. She relies on basically jumping in the direction she wants to go, or the crowd pushing her the right way. Maybe if she pulls down one of her balloons and holds it closer to her, she could change the balloons’ angle of attack while jumping. Strategically let go of that balloon to make its line go slack for a split second, letting her drop a few feet. That’s probably the extent of Valentyna’s control.

If anyone in the theatre has an air vortex cannon, they could control Valentyna’s flight a lot more than she could.