Entire barrages of missiles or torpedoes can go unchecked by point defense!
My response would be this:
Barrage combat is not intended to be something magic that has properties all of its own. It is a shorthand mechanic for working out volleys of fire from dozens or even hundreds of weapon mounts with a single dice roll.
The end result for a 4800-torpedo-long-6 barrage should therefore more or less resemble the results if I could be arsed to work out the point defence and attack rolls for eight hundred nuclear torpedoes longhand.
In 'standard' combat, point defence is a reaction against
all incoming missiles in a turn - you keep shooting until you miss. So I'm fine with a 'point defence' reaction giving you coverage against each barrage.
One thing I would say - although this is less solid - is that if dodging and firing lasers takes two reactions, then as far as I'm concerned, firing sandcasters takes a third - "fire sand" is a seperate reaction type. I know you're using the sand for point defence, but what you're doing to achieve this hasn't changed. Different gunners and different fire control doctrines will be required.
The other response - which requires a very strict reading of the rules - is the opinion of a couple of former players I know:
A target may react to incoming missiles by dodging or point defence. This reaction does not take place until the turn the missiles arrive at their destination, so any manoeuvring or shooting must wait until then.
Refers to a single reaction, and explicitely says
or, implying you do not have the option of
and.