Which is more complicated for the average user than just logging onto a web page on the device to set it up. I don't think my post mentioned a method of set up at all.
IP67 requires a unit to be capable of being submerged in a metre of water for half an hour. That's not a likely scenario for an access point. There is no need to install the UAP-AC-M in a sheltered location provided that it is upright.
Actually, no, but if you do this it must still be earthed in the summer house, as I've stated. The device can be earthed through its power supply but only if the the cable and connectors are screened. If there's no screen, it's not earthed.
One reason for it to be earthed is that installing it without an earth invalidates the warranty;
As I suggested, you can buy it by the metre and it won't break anyone's budget. Broadbanduyer charges £0.83 a metre including VAT and MS Distribution £0.52.
if you have an alternative source of shielded outdoor grade ethernet cable sold by the metre, that's good too, but apart from the Broadbandbuyer and MS Distribution, I don't know of one. It's worth pointing out that gel-filled ethernet cable, which you recommend, can be more expensive than regular outdoor grade cable. MS Distribution lists Tough Cable Pro for £0.52 per metre but gel filled cable costs £1.03 per metre. You also mustn't use it indoors because the gel is flammable - (the same warning that applies to BT CW1128 gel filled telephone cable).
Polyethylene jacketed cable is suitable for continuous immersion and can be duct rated whether gel filled or not. The reason some PE cables have a gel filling is to prevent water ingress should the jacket become mechanically damaged.