Might not have even been that deliberate on her part. The border conflicts could have been symptomatic of local kaldorei princes and lords seeking to expand their own holdings for reasons of prestige and/or political clout, instigating a response from the trolls.
Then the Zandalari went to Azshara and reached an accommodation of telling their respective peoples to both knock it off before things could escalate into a full-blown dedicated war between empires that the trolls probably couldn’t win and Azshara wasn’t particularly interested in pursuing anyway.
When it comes down to it, most of Azeroth’s mortal races - even the most aggressive and bloodthirsty ones - haven’t generally seemed very interested in conquering and ruling other races. The jungle trolls enslaving goblins was something of an anomaly in that regard, and the mogu are a rare exception, being the only major power in Azeroth’s history that was known to have subjugated and ruled over numerous other races outside their own.
Rather, when conflicts arise they tend to seek to either drive out their enemies or wipe them out. Conquering them and making them subjects rarely seems to be treated as an option.
All that said, the fact that the high elves basically managed to avoid conflict with the Amani up until they “crossed the mountains” (implying either Alterac or the Hinterlands, given those are the only mountain range-type areas in the northern EK recognized enough by lore to not just be zone-demarcating game mechanics) it seems like maybe after the Sundering ripped half their territory out from under them the troll empires’ populations might have generally fled east to seek protection from the ensuing chaos in and around the capital.
And since they’d just lost large chunks of populations to the bottom of the sea that had previously spent millennia growing to occupy their territories, the troll empires may have just not recovered yet to the point of actually reclaiming and resettling as far west as Tirisfal. After all, the high elves are described as having not actually encountered the trolls until they had crossed the mountains and begun to approach their goal (i.e. the peninsula where Quel’thalas would come to be. So at the very least they’d left Tirisfal behind, and since the Alterac Mountains are the only mountain range that’s been deemed important enough to name in the northern EK subcontinent, it’s not unreasonable to presume that may be where they crossed over before alerting the trolls to their approach. (Plus unlike the squared-off barrier mountains in-game, the Alterac Mountains are the only ones in that part of the world that are given a mountainous climate to go with their peaks., which fits with the horrendous weather the high elves contended with that killed a bunch of them.
The whole world reeled from the Sundering for a really long time, with the notable exception being the Gurubashi using Hakkar’s power to accelerate their recovery (and then cause their eventual implosion). Places like Tirisfal, Silverpine, Alterac and perhaps even Hillsbrad were likely only sparsely populated by the trolls, or the vrykul and humans would have had a much harder time of it when they landed there as what were effectively refugees.
Something to consider as far as the shifting geography of Zul’Aman is that it’s possible what we call by that name today may have just been the temple district of the original city. Like the equivalent of what Gundrak is to the whole of Zul’Drak. After all, when it comes to sheer territorial loss it comes across like the Amani and Gurubashi lost a lot more than the Drakkari, not only in territory taken by the night elves but then landmass literally dragged to the ocean bottom by the Sundering. Remember, like the kaldorei these empires had thousands of years to grow, meaning their capitals were likely pretty massively sprawling metropolises at their heights with gigantic populations. Similarly, what we consider Zul’Gurub now could be the same sort of thing, with the rest of the original city sprawl having been lost to the wilds or parsed out as ruins-dotted villages between the smaller warring tribes. Plus on top of losses to the Sundering, there’s that whole story in-game about a massive portion of Zul’Gurub’s cityscape being drowned by the seas during some conflict between the Gurubashi and the servants of Neptulon. Between all that and the Hakkari civil war, it’s hardly a wonder the Gurubashi have even fewer recognizable ruins left than the Amani.
Just overall though, it wouldn’t be surprising if many of the formerly troll-held lands that were fairly distant from the capitals - while technically theirs - still hadn’t been physically retaken, resettled and made defensible yet even by the time the vrykul and humans, then the high elves made landfall. The Sundering was insanely catastrophic and damaging to every civilization living on Azeroth at the time (except the pandaren 'cuz mist) and it took the world’s surviving nations a really long time to regain their footing and acclimate to the general environmental upheaval that followed. Arguably none of them really did. In 10,000 years one would expect a pretty insane population rebound, but neither the trolls nor the elves ever truly bounced back. In fact the notable exceptions of the Gurubashi and the night elves at least managing to rebuild new societies capable of sustainable growth were in no small part thanks to powerful immortal agencies overtly stepping in and lending a hand (albeit in a manner that proved ultimately self-destructive in the case of the Gurubashi.)
