Trees will normally break dormancy by May, but can sometimes be very late and still only starting in leaf in June if the spring is very cold and wet. If in doubt whether the tree is alive knick a small amount of bark and if green below then the tree is still alive and should not be harmed as it may just be slow to break bud. A further test is if the twigs are still flexible, if so then they are still alive.

You should record the tree and send photos to Forest Research via TreeAlert. Whilst they focus on forestry trees they should be happy to look at fruit trees as well: https://treealert.forestresearch.gov.uk/

Brown leaves usually (unfortunately) means fireblight https://www.rhs.org.uk/disease/fireblight).

Not good I’m afraid. This is no longer notifiable as it’s so prevalent. If trees get branches with these sorts of leaves then cut the branches back to good wood and burn the branches. Having started with the probably worst case, the problem may be less severe. See https://www.rhs.org.uk/fruit/apples/tree-problems-faqs

The first thing to check is the bark at the base of the tree, if this has been stripped by some creature (rabbits, sheep and beaver are all very partial to fruit tree bark) then this could have killed the tree by ‘ring-barking’ it. Are all the dead trees adjacent to each other? If so, then the cause is possibly more localised and could be something in the soil such as waterlogging or pollution such as spilt oil, diesel or weedkiller.