It does definitely get through. When I first encountered this, I kept trying to submit my question. When a reply came through it was evident the question had got through each time. Trust it's got there and keep checking whether you have a new agents message on your 'my giffgaff' page. Sometimes they let you know you have a new message by emailing you and sometimes they don't.
Yes it does, I have sent several messages that have been received despite the 404.
I check by selecting back and then refresh on the page. You can see your response in the trail.
Yes, you are right, if I look at the support trail (which i can see as its a response to a previous query), I can see my update.