So for the older laptops, you are going to need a VGA cable which will then plug into the projector. So you will have two cables running from the projector.
I feel the speakers will be inadequate for a room size and you should be looking
My suggestion would be to look at some PA speakers and maybe a bluetooth solution like the
SubZero SZS-P8A which would allow you to connect the laptops via bluetooth (it will require them to connect and be within a certain distance in order for it to work). They also have a pair of RCA connections which for laptops would need a
3.5mm to RCA which would use the headphone output
Using something similar to the above should ensure you have enough power to fill the room at a respectable level without causing long term damage as those speakers are not going to last long if pushed which could also shorten the life of the Yamaha to
If you wish to stay with the electronics, you could attach headphone pre-amp like Pro-Ject's Headphone S2 box, but this will just a sticking plaster and long term, you will find you will have issues with those speakers if the hall is large as you will be running the Yamaha at a very high level just to create an audible level
So, I think you are correct that you haven't been fully understood when explaining your situation as they shouldn't have sold you the small speakers. But lets see if they know how to connect these laptops to both devices without causing long term damage!
I know I used to assist a few cinema clubs and they never really had any issues with my recommendations, but they did use a DVD player which would have worked perfectly with the Yamaha although I would never have recommended the speakers as they are far to small and you are just waiting for them to break