According to the HMRC's guidance notes, which you can read here, stocks and shares ISAs can include:
Yes. AIM shares have been permitted ISA investments since August 2013. As always, it is important to check with your broker for any restrictions.
You should consult with your broker on the specifics given the number of exchanges involved but basically, yes, to the extent that they are traded on a Recognised Investment Exchange. Exclusions and restrictions apply depending on whether the exchange in question is ‘primary’ or ‘secondary’.
Usually yes but you'd be best off asking the fund provider or your broker, to be sure. Apparently, most ETFs listed in London are domiciled in Dublin or Luxembourg, which means they are classed as offshore funds, and some might not benefit from the tax advantages of an ISA. It's worth checking on the ETF provider's fact sheet and other literature to see if this is the case.