Answer to number 1:Concentration and Interrupted Spells pg. 331 wrote:Normally, you can concentrate even in a distracting situation, but if you’re casting a spell and you take damage from either a successful attack that targeted your AC or from an effect that you failed a saving throw against, the spell fails.
2. Since your Readied Action is not purely defensive, it resolves after they already cast their spell (assuming it isn't a 1 round cast), so it would not disrupt them.
2. Attacks of opportunity resolve before the triggering action, as a specific exemption to the normal order of operations (page 249). Additionally, you can specify to ready an action for when a foe begins to cast a spell (page 331). In this case the trigger is when the target starts casting, in which case you are allowed your readied action immediately afterwards, which occurs before the casting would be complete.
Attacks of opportunity are always resolved before the action that triggers them.
You are most at risk of taking damage while casting when a spell’s casting time is 1 round or longer, you have provoked an attack of opportunity, or a foe readied an action to attack you when you began to cast. However, if you are taking ongoing damage (such as if you are bleeding or on fire), your spells are not disrupted in this way.