I love
Magnus - not a surprise since I'm in love with Scandinavian names lately. I would use it even as a firt name, but I'm not sure it would match your naming style. As a mn, it's perfect.
I like
Joel MagnusFarrell Magnus
Lorcan Magnus somehow is a mismatch to me. I would like
Nichol Magnus, but
Nichol sounds too feminine for my taste (it reminds me of
Nicole too much), and
Carwyn is such a unique name that I would use a more common mn.
Sing once again with me our strange duet
My power over you grows stronger yet.(The Phantom Of The Opera)