yes all the changes are post 6 i.e. less for lane phase.
maybe you are correct. here is my opinions on your various scenarios:
poke: most pokle from mid-lane champions cannot be sustained by supports on their mana pool, specifically if the support or the carry takes a hit. This on account of a mid-laner's poke just hurting too much. poking back is taking a risk which is greater when: 1) your range is low, 2) your poke pattern is unsafe (lulu can EQ combo, sona has to walk to aa range) 3) you're a catch target (one of the champions the other team is happy engaging on) 4) your poke is weak (no threat level = they are happy trading your poke for their poke or for their engage).
siege: you don't have no wave clear so your mana pool isn't working on that front. your poke is dangerous see above so you can't do that either and your sustain vs actual mid-lane champion's poke is till going to be garbage.
skirmish: requires "cc poke". I.E. the ability to constantly put your team out of reach of opponents with repeated CC harass as you skirt the exterior of the fight. I am not sure how strong is sona's kit, specifically her E uptime / strength and E passive uptime / strength - when compared to the opposing support (in this scenario you are in direct competition over who's more disruptive).
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ofc i see that on level 16 the CDR on her kit becomes completely absurd and she becomes "urf sona". Not sure if I like that idea but we all know that urf sona is broken so maybe you're right and it's simply a case of sona now having a weak midgame for a monster late game.