Managing several Sophos UTMs and Sophos XGs both at work and at some home locations, dedicated to continuously improve IT-security and feeling well helping others with their IT-security challenges.
Sometimes I post some useful tips on my blog, see blog.pijnappels.eu/category/sophos/ for Sophos related posts.
PS For future participation, an unwritten rule here is one topic per thread, but we give newbies a break! [;)]
[Netgear]---[UTM]--[8-port switch]--[Internal devices]
And, you're right that you can just take the Netgear out of line and the UTM should grab a public IP from the modem.
The usual recommendation with a wireless router is to disable DHCP and tape over the WAN port so that you can use it as a wireless switch behind the UTM. When you're ready to do that, I bet there's a way to do a factory reset of the Netgear so that it has the default credentials for the admin account.
Cheers - Bob