Hi,
1. You should not need any tax rule in that case. Just make sure that no tax rules apply to the channel islands zone and no tax will be charged.
2. The post code option can contain regex. So yes, you can do almost anything with it. However, the syntax of regexs needs to be followed:
www.regular-expressions.info/tutorial.html
3. It depends. If the "cumulative" setting of your tax rules is activated, several tax rules can match. If not, only one tax rule can match, and thus, once one match, its tax rate is applied and the other tax rules are ignored.