We use the X18 (also operated by yours truly) and only 2 of us on in-ears, My 2p:
- I want to hear everything including the punters, IEMs will be too isolating - Ambient Mic will work fine. Although I don't find the isolation half as bad as some people imagine it'll be.
- What do we do if the iPad crashes or WiFi stops working? - I only use it with a wired connection to a laptop. If you're dead set on wireless, then get an external router, the built in one isn't very good.
- On having an app to control a personal monitor mix: I’ll never be able to do it, too complicated. (From the singer, who has the biggest objections to IEMs, but constantly wanting ‘more me’ and causing feedback with his mic antics) - You can setup his mix for him initially. A few minor tweaks later, you'll probably find he stops asking at all from gig to gig.
This can also be done these days on a relatively modest budget. I'd suggest hard-wired IEM at first, as decent Wireless IEM units are not cheap. We use the Behringer P1's, but the newer, smaller P2 units are also good and cost about 30-odd quid each. As far as earphone themselves, the KZ ZS10's seem very much in favour for those wanting to dip a toe into IEM's and can also be got for 30-odd quid.