A Place To Experiment!

[namida]

If the splatpad and updraft overlap, I'm not sure which takes priority. A splatpad below an updraft would work as normal.

[WillLem]

What happens when you put a splat pad into an updraft and then have terrain go through it?

I've attached a level which I think tests what you're asking - feel free to edit it accordingly if it's not quite what you mean. I have the Lemming on the left walk up a diagonal slope which takes him upwards through an updraft/splatpad combo - there is also a walker so he can turn and walk back through it. Also, there is an updraft/splatpad combo in the gap; platforming through it doesn't seem to have any effect...

Or does the updraft simply always overwrite any "splat" characteristics

As shown in the level - splatpads override updrafts! This is the case no matter which is "in front".

If the splatpad and updraft overlap, I'm not sure which takes priority. A splatpad below an updraft would work as normal.

Conversely, anti-splat pads override splat pads! Again, this is the case no matter the order of the objects. The Lemming on the right falls onto a splat/anti-splatpad combo, with the spatpad in front. They fall safely!

[namida]

It makes sense that one takes priority regardless of object order. NeoLemmix's object detection basically checks "Is there an object of type A here? If so, handle it. If not, check - is there an object of type B here? If so, handle it. If not, check - is there an object of type C here?" and so on, until every type is checked for.

NL only checks for the individual object, if it's an object type where the individual object matters. So for example - if NL detects "there's a fire object here", the lemming dies - NL never makes any effort to figure out which fire object it is, because this doesn't matter, a fire object will always kill a lemming, so the object order doesn't matter. At the start of the level, NL has already figured out "this pixel is covered by a fire object" and that's all it pays attention to from there on. On the other hand, if NL detects "there's a triggered trap here", NL must then figure out exactly which trap it is (because firstly, it must check the trap isn't already in use, and secondly, it must animate the trap). Only at this point does the order of objects become relevant.