I am looking at switching from Spark to 2Degrees for my broadband/landline connection. This was triggered by the imminent loss of the included Lightbox subscription. On the face of it I will save $30 a month for an equivalent service, gain free national landline calls, and a 12 month Amazon Prime subscription.

I have a few questions however. I am located in rural Whangarei, on an ADSL2 connection ~2.5km from the cabinet.
For an ADSL connection the speed is excellent and consistently achieves 10-13Mbps on Speedtest (ADSL stats - Maximum upstream rate (kbit/s): 1224, Maximum downstream rate (kbit/s): 17756). 
I assume nothing will change physically with the ADSL connection however I'm unsure where the Internet connection will change from Chorus/Spark to Chorus/2degrees, and whether there is a possibility of performance issues, ie in the backhaul from the cabinet if this is allocated by retailer for example?

Am I correct in assuming that the Home Plus landline performs fine over ADSL and uses some sort of QoS so as not to be affected by our Internet usage? 
Also does 2Degrees provide any QoS for their mobile Wifi Calling when its via their broadband?

I have a shed about 50m away with a phone extension from the house, and wifi access wired back to the house. I was going to cut the phone output at the master filter and feed the extensions from the modem SIP phone connection, however I saw a post from 2018 where it appears you can setup your own SIP client on a mobile phone and connect via the home wifi. https://www.geekzone.co.nz/forums.asp?forumid=85&topicid=239919
Is it possible to have the modem SIP connection and 2 mobile phone SIP clients connected simultaneously on wifi, so they all ring when there is an incoming call? If this is the case I won't need to alter the physical wiring.

Does 2Degrees allow you to renew the 12 month contract to gain a further 12 months of Prime, assuming the same offering is still available?
How is the switching process handled and is downtime minimised?