What should I infuse my blm with?

[Mhaeric]

As for BLM and piety, there are certain piety tiers that you can meet to be able to, in order of cumulative ascending piety tiers:

1. squeeze out an extra Fire IV before needing to switch to umbral ice
2. make it so that you always have enough mp to be able to thunder after blizzard 3 without sometimes needing to wait for a tick
3. make it so that you always have enough mp to be able to blizzard 4 immediately after blizzard 3 without sometimes needing to wait for a tick

The first requires only a very tiny amount of piety unless you're a dunesfolk lalafell who don't need to have any extra at all. The others require a moderate piety investment (~40 or so) in order to achieve. The easiest way to achieve this is to put it on your Anima weapon, since then you can save all your melds for other stats. This also makes you less tied to having to have piety on your SMN if you play both and don't want it there.

Beyond that, sps is your best bet since it helps with mobility and being able to dodge quite a lot on top of increasing your rate of fire which is important as a slow casting turret. After that, technically I think det is better than crit by a slight margin, since it always applies it's effects whereas crit's benefits only apply occasionally due to the slow attack rate... but det is boring. Since the margin of difference is so small between the two, most blm I know just choose crit as the secondary stat for the fun factor.

As for melding now... I would wait the 3 weeks until 4.0 hits, since all your current gear will become obsolete almost instantly.

(As for the SMN feud, I don't know Neela but I would take anything Silverquick says about summoner with a grain of salt. I agree with Neela here that Piety is not a particularly useful summoner stat to have when weighted against what crit and sps bring to the table.)

I will get 3-4 Ruins3 and more potent DoTs.

Don't forget more pet damage, since their attacks are directly improved by sps.