Hi! First of all thank you for your feedback!
Ortho4xp - this program generates mesh and builds phototerrain. If both are active, there are 2 custom meshes - our (from the scenery) and your (generated in ortho4xp). However x-plane can load only one custom scenery. If ortho4xp loads first in x-plane (line with ortho4xp higer in scenery_packs.ini) you have troubles with floating buildings. If our mesh load first all work correct, but your otrho4xp prototerrain is absent.
Our mesh works correctly if you use the checkbox in x-plane settings “runways follow terrain”, and your custom meshes (ApolitX HD meshes, or generated Ortho4xp) will not replace our mesh from Seattle Airports XP scenery.
You can try to regenerate mesh ortho4xp with custom dem (I do not know whether it will or not be possible, i never use ortho4xp) like this
https://youtu.be/sU-PA0BodJU .
About blurry signs - you probably have very low texture resolution settings in x-plane. Increase them.
I hope we find solution together!