They'd probably need special insurance for this - doubt their day-to-day policy would cover something like this.

Might be worth it outside of normal store hours, but 15k is not going to be enough to basically shut the place down for an a few hours. Worse their liability would likely be a significant issue.

Yes, but that makes Ikea assume some responsibility. At the end of the day Ikea sells furniture, people playing a massive hide and seek game are not there to buy furniture.

They could use waiver forms, like any other place hosting moderately "dangerous" things (like trampolines, etc.).

That said, you could still allow people who came to actually buy furniture.

Though, I think it would require more preparation than just winging it.

Waivers aren't – just like anything else – perfect protection against (legal) liability. In the U.S. at least, everything can be litigated and even just defending one's self can be expensive.

this! and they could limit # of participants so it doesn't get out of hand.