1) Keywords that have specific rules in place are themselves effects. Effects are not restricted to the 'effect' line of a power. If a power does something, it's an effect. If a power's keyword does something, it's an effect. It does not even have to be a power, if a feat, trait, feature, or any other aspect of the rules does something to something else, it is an effect.
So the pertinent question is: Does the Summoning keyword do something to the summoned creature? The answer here is no. The Summoning keyword does not forbid the creature from taking actions, it merely states that the creature has no actions of its own.
That's a distinct difference, having no actions is not the same thing as being forbidden from taking actions.
Therefore, summonings can flank.
What most summonings do not have are basic attacks, and that is why they cannot inherently make opportunity attacks, unless the power that creates it has an opportunity attack as part of the creature's ensemble.
Does attacking through a summons provoke Opportunity Attacks on the caster?
You're using the summoning power which is listed as a 'Ranged' power, but it's the summons making the attack using your stats, so it seems like it shouldn't.
I think the way the summon rules phrase things it's definitley the case that you do not provoke OA by attacking with them, but I'm amazed there is not a clear sentence in the rules somewhere that says, "Although the act of summoning a creature is a Ranged power that provokes Opportunity Attacks, attacking with a summons is a melee attack by the creature that does not provoke Opportunity Attacks for the caster."
That entirely depends on the power the summoning is using, doesn't it?
Let's take the case of Summon Blade Angel, Invoker 9, PHB2.
It is itself a ranged 5 power, so normally any attacks it makes would be ranged attacks. However, the actual attacks listed say 'Melee' as clear as day on them. This contradicts the Ranged attack-type of the Summon Blade Angel power, and as a specific case, therefore supercedes it.
Thusly, that attack is a melee attack, and melee attacks do not trigger opportunity attacks.
'Okay, Draco, Mister Smart Guy, what if it's a ranged or area attack?'
Well, firstly, the summoned creature will, itself, trigger an opportunity attack. It is making a ranged/area attack, it triggers an OA. This is pretty simple.
HOWEVER:
YOU also trigger an OA. This is because the summoning has you make the attack. Because you are now making a ranged/area attack, you are also now triggering OAs.
'What, so you're saying we BOTH trigger OAs?'
Yes I am. The summoning is making an attack by the rules, and any time it makes an attack, you are also making an attack by the rules. As you are both making an attack, it only makes sense you both trigger.