You may have gathered some textures and data that is not needed and would only be cleared if you close Blender and reload, but there is a way, in the outliner change to "Orphan Data" view:
There you should find a list of all the data, textures and such that are not used and with a click on Purge.
You are able to clear unneeded data.
The File Browser preview only shows solid view colors.
You have three options here:
Adjust the solid view color for each material (in Material tab)
Set solid view color for each object (in Object tab)
Use random color for each object
You can switch between different modes using the Viewport Shading menu.
Seams are something Blender uses internally to let it know where you want to cut. They cannot be exchanged between different programms. In fact internally they don't exist at all and those vertices are actully duplicated, since no vertex can have more than 1 position in UV Space.
I'm not sure why you would need them though, since your mesh is already ...