Get a ballistic helmet, if you cant afford or find one, get a motorcycle helmet or even a construction hardhat. The airsoft larp gear will just get you hurt.
Also, and this goes for ballistic plates too, if it ain't NIJ certified it is worthless and will get you killed or injured.
https://www.hardheadveterans.com/blogs/reviews/bulletproof-helmets-what-you-need-to-know
Motorcycle helmets are usually good for 5 years. The foam can lose some of its' integrity. That said, I would think you could probably protect yourself relatively well against impacts regardless, but it's something to be aware of. Look for a helmet that is DOT rated and I think it should be alright. There are some real cheap ass helmets out there that I would never recommend riding in, but I would think would be OK for impacts and the like you might experience at a demonstration.
However, these will definitely give you zero ballistic protection. Luckily rubber bullets and paintballs don't have the velocity of live rounds.
DOT isn’t a very rigorous standard, better to go with a Snell rating.
Either way a motorcycle helmet should be replaced after an impact.
My opinion is that using a motorcycle helmet shouldn't be a first resort, so in that sense I wouldn't recommend shelling out the extra for a Snell helmet. However, if you ride and also want to riot it up, go for it. That's the main reason I left the bar lower at DOT, because you can find DOT rated helmets under $100 usually. Snell, probably not. But yes, it should definitely be replaced after impact, at least if your head is inside.