I think it will be easier to use than you say. The 120mm base makes it much easier for it to be near a model at the end of it's movement, though it does mean that a model hiding directly behind it will use it less.
Will likely find a lot of use after a jack has gone in. Move the trident up behind jack to back the jack up 2" to get things out of threat range of enemy charges. Combine that qith something like rhythm of war and a jack could charge in, kill a heavy and end the round 5" further back than where it had to be to kill the jack.
The new version is a lot more restrictive on the placement effect, and it will be a bit trickier to use Telekinetic Wave and get optimal use of your shooting on the same turn. To me at least this means TW will become fairly cornercase and I probably won't count on it too much when list building.
tbh I always interpreted that final CID writing as the final writing in the card database, but you are right in saying it is more restrictive.
has slipstream ever been anything other than a corner case utility thing though? it's not like seraphs have dominated every legion list as far as beasts go (I've never seen one outside of oracles, although admittedly none of the legion players I've faced were big into hyper-competitive things), and while she's new skarre3 didn't exactly completely sweep in and take over the cryx meta despite having access to shipstream.
the main draw to the trident for me was a colossal base with coordinated movement that fit into more themes than DoI. that's a lot of LOS blocking and screening available to ret's generally squishy models that can then still see and move through it when required. TW was never more than just a situational bonus utility thing stuck on an otherwise fairly durable, fast, but inconsistent in offensive output model.
Well Slipstream has always been a staple in Legion for the threat range game (especially in MK2 and early MK3), you just don't see it as much nowadays because the seraph itself is overcosted and nobody wants to pay for it except in a beast heavy lists. Couple that with the fact that PT is very good right now and OoA has taken a backseat in most legion pairings.
I think you guys are right though and I may have dismissed the Trident's place effect too quickly. You just can't charge something in the distance and still get the place (unless the lines have already closed as isotope mentionned), but the option to stay a bit back, place a jack and shoot what you can is going to be a good play in many cases.
Can't wait to put that thing on the table to get a better idea!