I don't think it's possible to give a survival rate, because as with all these things 'it depends'.
Young saplings would struggle if the weeds are taller than they are, on the other hand they are cheaper than older trees and tend to establish better in the right conditions.
You would need to prepare each planting hole carefully, and include some manure.
You would absolutely have to protect the trunks from rodents and rabbits, and if there are deer then your project is doomed to failure - they'll have the lot, especially when there's a whole orchard for them to plunder.
I see the brambles as being a problem - but think of all those apple and bramble pies
. A tree trying to grow through brambles would simply be swamped and strangled, and anyway you wouldn't be able to harvest them. I know you have said you would clear them, but they always come back.....
People are a possible problem too - if your patch looks really natural then people will think the fruit is theirs for the taking, or scrumping. Even if they knew it was planted, but you were not there often, the fruit would still vanish.
Sorry that sounds a bit negative. You will have a greater chance of success if you can visit fairly frequently to keep the weeds down until your trees are established.