Which mower are you planning on?
I've got a Husqvarna 420 so i can only offer advise based on that.
I've got 1 guide going into my orchard which only has a 2m wide entrance with 1 guidewire going in the middle of that, that works fine and never had an issue. You can set the guide corridor width in the software, i think its 60cm by default. It also only ever drives on one side o the guide (i think left from memory looking from the base station) so bear that in mind when you lay the guide down a narrow section.
The other option is to set it to follow the boundary wire for a sent amount of metres from leaving the base station to get you into other areas, that way you can get away with getting a model which only supports 1 guide wire.
As for bushes and trees, anything larger than say 3cm trunk i wouldn't bother islanding off, only do it for small bushes imo. I went round all my fruit trees and bushes in my orchard and in retrospect i only really needed to go round the bushes as now the trees have been in a couple of years are more than big enough to deal with the bumps it just leaves more un-mowed grass around their base and needs to be. You can also add more islands in later on its not big deal, i've done it several times now.
As for laying the boundary it's not as terrible as you think, i've done pretty bit stretches in not much time. When mine was installed i got the installation included in the price but it only took the guy about 2 hours to do my whole garden which is probably a bit bigger than yours i'd guess? Mine is 1/2 an acre but with a separate orchard which meant a lot more wire but i think im still probably only around 350ish metres
I'd be tempted to have it where the light blue line goes, as the you can set guide 1 to go round the front, and then follor the base right for however many meters to get into your back garden. It'll always find the base by picking up the guide in the end, you just might need to turn down the radio signal from the base so it ignores that and always follows the wire back to it.
However the other location would be fine too and lay the guide into the narrow bit and tell it to follow the boundary to the front garden.
Either way you're garden isn't very complicated so i'm sure it'll be fine, how many m2 are you looking at?