Definitive Answer: Does heal and shield power stack?

I think on items with the unique passive to increase heal/shield (censer, redemption, etc), it counts for all items, so only one unique passive is included. I don't think windspeakers counts though, so that and a redemtion/censer would stack, but a redemption/censer would not stack. So i would suggest getting either censer/redemption, and then getting that dank

They all stack, just not with themselves. So ardent+redemption gives 20% heal/shield power, but ardent+ardent gives only 10% heal/shield power.

You're generally going to want the heal/shield items for the sake of their other effects anyway, not to mention they're much more affordable on a support budget. AP will potentially produce bigger heals/shields at full build on the majority of champs with heals/shields, but tunneling too much on maximizing your heals/shield instead of overall utility is generally unwise.

+ Ap = op hp+++

If you stack AP on Soraka - you are giving yourself more stability because you aren't killing yourself - but allow yourself to be killed more easily by the enemy team if you do not build HP/RES.