I'm pretty excited about this event building, although the 'switchable production' aspect of it does seem a bit useless to me. This is a seeds-producer, and using it any other way just strikes me as silly.
My numbers for my city (Ch15) look like this:
When Producing Seeds: Saves 14 squares from Pop, 1.43 from Culture, 35.78 from Seeds production for total = 51.20 squares (or 2.65 Festival Merchants).
When Producing S2 Goods: Saves 14 squares from Pop, 5.56 from Culture, 36.04 from S2 production for total = 55.60 squares (or 0.76 S2 Manufactories).
When Producing Vallorian Guards: Saves 14 squares from Pop, -15.54 from Culture, 62.29 from Vallorian Valor for total = 60.75 squares (or 5.79 Vallorian Valors).
On the face of it, it seems like using the building for S2 or VG would be a good idea (it saves more squares, right?), but I don't think it really is. For S2 production, using 1 MM spell on a manufactory outproduces this building; so unless you constantly keep all of your S2 production buildings MM'd already (and still are lacking S2 goods), you're probably better off spending the enchantment than using this building to produce S2. For Vallorian Guards, a handful of supplies and a 5-hour time boost will give you as much VG unit production as this building will. Because those things (supplies, time boosts, enchantments) are quite a bit easier to come by than seeds, it seems like seeds really is the best option with this building. But would love to hear thoughts from others as well!