Here a (maybe thick ) thought
Is it at all possible to copy the ne Highmap data into the current province esps like Summerset,.. etc that way the likes of StarX maybe just have a lot of adjustments to suit new land levels etc
and still be able to use the current master esp along with the new highmap?
The problem is the Form IDs. Every single entity in an ESP has a unique Form ID within that ESP - but it's not unique to any other ESP (the first two characters in the Form ID are dynamic, and change depending upon your load order, but the rest of the Form ID could be identical to one in another ESP). When you merge ESPs together, any matching Form ID found in both ESPs causes a conflict problem, which is resolved by the ID of one being changed to a new ID in the resulting merge. Now if a mod (such as StarX's) places objects into a cell created by a master ESP from Tamriel Heightmaps, when Onra takes the Tamriel Heightmaps ESPs and merges them all together, there's a fair chance that the Form ID for the cell(s) used by the dependant mod will change. This will then break the other mod, as only persistent-placed objects will appear in-game (the non-persistent objects having been placed in a cell whose Form ID no longer exists).
Now I've already suggested two possible ways for StarX to avoid problems, but there's also a couple of things Onra could do at his end to avoid problems instead.
1) When merging, Onra can ensure that the ESP StarX is dependant on is highest priority, so that ESP's Form IDs will be retained and the IDs from any other ESP will be changed.
Problem here is that StarX isn't the only modder using Tamriel Heightmaps, other modders are using different Heightmap ESPs - they cannot ALL be set to highest priority.
2) Before merging, each modder using a Heightmap can send their ESP (no resources needed, just the ESP) to Onra (and stop working on their mod briefly). Onra can then use TES4Edit to fix the Form ID range in each Heightmap ESP to a unique range, while also having everyone else's ESP loaded into TES4Edit (so that they will be updated to match the new Form ID range for the cells from the Heightmaps ESPs). Once this is done, Onra can merge the Heightmaps together and release (removing the old separate ESPs from public access) and send back each modder's updated ESPs for them to resume working on (using the newly merged single Heightmap ESP & separate Region-related ESPs).
Problem here is ensuring that Onra receives the ESP(s) from every modder currently working dependant on the Tamriel Heightmap ESPs.
Option two is the best route, in my opinion - it's probably the easiest (especially for everyone who isn't Onra, since he'd be the one doing all the work) and it's certainly the tidiest.
But if Onra is unable to or not willing to do either of these options, it will be down to the modders such as StarX to try what I suggested earlier once the new merged Heightmap ESP is released.