It takes 2 minutes and 12 seconds for meki to regen as much mana as sapph crystal gives you.
Granted over a 5 minute laning period that is much more mana. However having that right off the bat for a potential gank-counter gank in river, etc, may potentially be better. and of course its also simply to build into fast cata, and then locket.
Dorans or mana manip are not particularly bad ideas, no. mana manip with an ap teammate, and someone with CC to protect you. dorans if you are 2v1 or other team has shaco/xin/garen, etc.
Aegis is a great item. Someone should get it every game. The problem is, getting it after 25 or 30 min, after your locket, ss or sury's is REALLY late. Someone, ideally your tank, should have gotten it already. IF the game is still going on, and no one else has done it. go for it. Absolutely.
But much like locket, its FAR better for the person that has it, then the team. So the tank/offtank should have it. and you are not the tank. (Although, it would go miles for increasing your surv. esp if you are getting focused)
SS is over-rated. Due to the 40% CDR cap, its aura is often mostly wasted. Nobody plans on it. Either they cap naturally with gear. As anyone that cares, gears for it. Or they don't care because they are a mostly AA champ.
Also, you really can't have too much mana. And the extra healing is not irrelevant at all. it keeps you constantly topped off. and with her escape tools, you can avoid being focused, and stay fighting. And keeping you alive, and keeping you in the fight, is providing your auras.
Why such a focus on wards? what else? Sona doesn't scale with anything but cdr particularly well. AP is ok. But not worth focusing on. So after 40% cdr, you are kindof left open. Auras, and more survival are you only real options. (Abyssal is probably better then SS. hell even starks.) Aegis should be in the hands of a true tank. If they aren't as I said, sure go for it. But all around, do you really need more survival? How are you dieing after you get locket? Your base hp isn't great. Locket gives a 430 boost. And has that extra heal, which shouldn't be underestimated. but w is a def boost and heal, e is a speed boost, and r is an aoe stun. 3 of your skills directly aid your survival. Additionally, all auras and skill have HUGE ranges. Meaning you can keep your distance, and stay well away from danger.
in fact, why do you even need flash with all your survival tools built in? Even Q lets you harass, and dps someone chasing you without targeting or stopping. in a team fight, how are the other team getting through your team, and your aoe stun, AND you popping E to speed boost yourself, and anyone else that needs a quick escape to kite, to focus fire you? Are you over-extending and not watching your back with, I don't know.. wards?
I run heal and clair on sona. Heal goes a long way when you don't have the raw direct heal of soroka/taric/nida. It directly synergies with all your def auras. Again, keeps YOU alive, and in the fight, as well as lets you heal your entire team. Dumb not to have heal. And clair again, mega team support.
You greatly underestimate the aoe heal on locket as well. 20hp/5s yes, is weak. Does nothing against focus. But often is enough, when someone evacs with your speed boost, to get them back into the fight to kill a tower after the fight is over. Especially x5. Being able to push lanes AFTER a fight wins games. 180hp aoe over the 40s a champ is dead and get back into the lane to fight again, shouldn't be underestimated.