Galloway is in a different party, but he was invited, knowing what he was. So, yeah, they are responsible for it, that was just boneheaded.
UKIP may have more members now than it did before its defeat in the last election; the sentiment for the UK leaving the EU may be there, though I doubt it. Meanwhile UKIP holds seats in the EU parliament (which is amusing) but has basically zero influence there. It has zero seats in the UK parliament. But yeah, people who say UKIP has peaked don’t know what they’re talking about.
Regardless, good luck in that campaign. There are members of Cameron’s cabinet who want out, even though Cameron himself probably couldn’t care less one way or the other at best, or opposes Brexit at worst.
If UKIP has more members now than last year, if it has more votes at the last election than it did at the one before, if had more MP's than it did the election before (only one but more than zero), if it has more MEP's now than it did in the last European election cycle, then surely if follows that its star is still rising. Your argument basically boils down to "UKIP aren't going anywhere, therefore for meaningfull change you need to vote for someone else", which basically is a self-fulfilling prophecy. I don't think UKIP has peaked at all, but I'm OK that all the clever people convince themselves it has. Victory will be all the sweeter.
Cameron has invested his political credit very heavily into staying in the EU.