You should go there ahead of time. Gather as much information as possible.
Try to get access to the room before the group arrives. Try asking the hotel employees.
Google the hotel, maybe they have a group shot of their employees or of a previous conference on their website.
You should definitely find out if this is seated or not.
If the people are not seated, all those 250 people have to be directed in some formation to take the image. This can require a change of location. There might be a big stair in front of or inside the hotel, which makes assembling groups (especially of that size) a lot easier.
Your YN 565 will probably be useless for the group shot. Getting an even distribution of light on 250 people with a single small flash will not work very well. But that shouldn't be a problem, because conference rooms are usually well equipped and it should be possible to light the entire room very well. Use this lighting for your group shot instead of your flash.
I am not sure how you come up with the idea of shopping for a new lens in this situation. With an appointment like this, how do you have time to familiarise yourself with new equipment? For candid photos your 50mm will work well. And there's no use for a f1.4 lens anyway. You are photographing a conference. that means many people interacting with each other. Even your candid shots would include at least a few people (2, 3, maybe up to 5) which makes using an aperture of 1.4 less than ideal, because you want everybody in focus and not just one persons right eye.
Your zoom lens will be a lot more versatile when walking around the conference, adjusting for different locations, where the "few steps back" that a prime sometimes requires, are often not available.
The flash will help in dark situations without relying on a wide open aperture.
The tripod can be useful for a staged (not seated) group shot. Keeping the camera in one place allows you to direct people better.
I don't see any use for the tamron, but keep it in the bag just in case.