Well, in this case it will use trample except against Elephants/Dragons. Trample w/ Size 6 is very powerful in itself, of course, but I would prefer just making area-effect swallow. However, I don't know whether it's possible to exclude self-hitting possibility in any way (& in differenence to great mace here it would be silly!

). Also I wouldn't make it so high-damage. There is weapon quality called swallow iirc, which multiplies damage against smaller beings. There is also a possibility to add small-area fire to its attack (it was mentioned but wasn't of great importance in Dune, of course!) - this would have an advantage as it would allow area effect with no actual chance of biting himself!

As for water weakness it can be simulated by using either cold or poison weakness. Or even both! Sailor's Death is iirc just a MRE negated damage (I'll look it up). What we could add to emphasize his vulnerability to water is the quality of Vampires & Air Queens which forbids them to enter water in any way (it's hidden iirc, but can be gained by copystats).
On stealthy I disagree. They were easily detected by tremors in the ground even while traveling deep below sands, and if they were not so deep they were preceded by huge sand waves!
On burrow... well, to duplicate no movement inhibition we can use mountain survival (waste one is a must) - and it's not as if Shai-Hulud could really survive beneath forests/swamps!

As for battlefield - yes, onebattlespell is probably the way if we want to include such surprise attack at all. Method for this is probably to make new spell called burrow or somesuch & copy to it blink, etc. with different description (if you think it needed).
I also don't know how to duplicate it giving off the spice - probably as gem producing?

Another thought just occurring to me is a possibility to somehow intertwine this with hashishiins' drug use...

And as Shai-hulud was highly mystical creature, it should get 3 misc slots!

Size 7 is probably impossible in the game, but you can either write to Kristopher & ask him, or just make it so & test whether this would cause crash/bugs... I for myself wouldn't make elephants largest possible objects in the game but programmers could...

I already said that I'd prefer him as a Pretender, so we should wait whether someone else has some ideas on this... BTW, did you mean commander-type, or troop-type summon?