League of Legends (LoL) Question: ¿Thoughts on Lux Mid-Support?

Hi people, I asked a time ago about Thresh-Leona builds, but since lux was free about 2 weeks ago I started to play support-mid with her. In mid she is a beast, and contrary to other Mages-AP assasins (like orianna-ziggs-zyra-ahri) I don't feel she is hard countered by now popular melee AP or AD mids (kass-kata-talon-zed-yasuo).

In mid with one big item and good warding she owns lane, I've even been like 8/1 on lane phase. What do you think about Mejai's Soulstealer? I haven't try it. It's obviously a snowball-cheap item but until full stacks it don't offers anything more than flat AP, and it seems impossible to me stack it up 20 times without dying.

As supp with a decent ADC I win lane at least 75% of the time. She has some troubles with tanky supports but If my adc is smart enough to don't get grab or stunned we win lane phase. The only problem I see with lux is that she needs some mana in order to keep poking. I usually start with spellthief, and buy a doran's ring+spellthiefe upgrade+wards or upgrade+faerie charm if not enough gold. I don't really use frost queen's active, so if I have both chalice and frot queen let's say by around minute 30, they both give similar stats, with the plus of the gp10 on frost and cdr on chalice. So should I upgrade up to frost queen and buy athene's, just one upgrade on gp10 item+áthene's, full frost queen and no athenes item? Both fully upgrade items are pretty much like the same but If I don't buy one of them I have to come back pretty often to base for mana, but also, If I upgrade them both I willhave some damage problems.

In overall, what are your thoughts on Lux support?