Hi  @TheChaser 

I have a similar-ish setup to you and have been playing around with my new Deco XE75's quite a bit at the moment and here is my experience;

1. My primary Deco (the one physically connected to my ISP router) is approximately 20m away from the satellite Deco in my shed. At this distance i get a very weak signal over wireless (approximately 4Mbs). In my case i have hard-wired from the primary Deco to this satellite in the shed, but i think if you are looking at just going wireless for this connectivity you may struggle to stream TV across this link if it will connect at all.

2. Speaking for the XE75's only...As long as your primary unit is the one connected to your ISP router then you can turn off the satellites and on they will automatically reconnect as required when powered back on...this is assuming that they are all in range of each other.

3. I am not aware of any option to turn off the wireless and be wired mode only. It would appear that the  software (app & web) is very limited in nature to keep a simplified user experience at the expense of more granular options.

If i am honest, i think that getting a signal to the end of your garden will be a challenge without any physical cabling. If it is just TV streaming that you want in your shed and don't want to install any cabling its probably worth looking at other options. When i first started out, i was using an old Sky dish to pick up free channels in my Shed, perhaps there are other options you can investigate?

G