I really think the spell suppression rather than dispelling is a change that needs to be made. My beef with beholders isn't that they're too tough, but that every time I fight them I have to recast all my buffs. We know its possible in DDO to surpress spells rather than dispel them from the Globes of Invulnerability mages can cast. This should be applied to beholders' antimagic cones.
And yes, according to PnP the eyebeams shouldn't function in the antimagic cone either, but I'm not sure that beholders need this type of nerf done to them. They're not really that hard, just annoying, and making them easier may or may not be the right move. I'm not sure if anything needs to be done about them firing into antimagic.