Not really sure about working in Multichannel mode to start.

I use it only to convert between RGB and CMYK and bypass the color mapping that happens normally when you just "switch modes". This middle step lets you convert between modes without recalculating the channels.

as to the question 1 ...the colour changed totally
When you switch modes (RGB to CMYK) Photoshop automatically maps the colors to compensate. CMYK relies more on pigment than light so it bumps up the contrast and adds a black channel to help with the shadows.

When you choose Multichannel, Photoshop changes your image so they no longer mix to create a full-color image but exist independently in the confines a single image. This transfers the exact contents of the RED channel to the CYAN channel, the contents of the GREEN channel to the MAGENTA channel and so on...

As I stated before, Im not so sure about starting in Multichannel as opposed to using it as an intermediary step between modes.

Eartho makes a good point about naming and color between apps. It's a VERY good habit to form.

sorry i don't think i was much help...

retrocity