Hi James,
Files were well-received one day ago and a full investigation was done.
Here is a summary for a quick update:
1. The system library for Inventor (Inventormateriallibrary.adsklib) was in unstable state, please do no write any customized assets to this library and the guideline as below, you can drag all of them to a new user library and replace with the right one enclosed to the right directory.
Enclosed the original Inventor material library here and to replace in the directory (take the default project as an example) -
C:\Users\Public\Documents\Autodesk\Inventor xxxx\Design Data\Materials
Please avoid saving any customized asset to system library which might corrupt the library and there is no way to repair.
Tips-
You'd better to create a customer library for any of customized assets. Regard to customized library (*.adsklib) file is compatible from release to release without migration process, which means, it can be open in higher version. If you want to share the library, then, Just make sure all clients are the same version of INVENTOR, you can also set the file as read only to lock it up, then, only the admin can provide modification to keep the library in good condition. BY the way, make a copy of this file would be a great backup plan for any time. Be sure to set the library as active library for both Material and appearance to make it works better in project setting.
2. Material migration process is based on Inventor library which means the process is comparing assets to create new assets to your new migrated library, so if Inventor material library were corrupted already which may lead to migration corrupted as well. so I've replace the healthy Inventor material in design data and migrate the xmls again, the new migrated library was enclosed as well.
3. Please create a new project and set the new migrated library as default for both Material and Appearance setting, make sure the option for Use style library = Read-Write.
4. Migrate template file to current version and then launch Task scheduler to update the template with update design task.
5. Style confliction dialog is design intents due to material migration, please ignore it.
6. Regard to save sheet metal rule, it shows as unable to save due to damaged sheetmetal rule xml file. If you replace with health Inventor material library in step 1, create new sheetmetal from the template, there is a warning as below which indicated that your original xml files migration do not contains those material assets, that's the case shows sheetmetal rule is not based on the newly migrated library at all.

From above steps, finally realized that it is not a migration case at all, it is just a case of corrupted sheetmetal rule xml.
So just guess, you had migrated material xmls to Inventor material library directly before and you make modification for those assets in Inventor material library which caused the Inventor material library corrupted (No thumbnails for the corrupted assets) already before. Because of the corruption, the sheet metal rule xml also corrupted, you have to manually recovere the sheet metal rule xml and reassign proper material for each rule and then save it, but it indeed a data corruption case, no luck to recover it finally. again, please do not put any customized asset to Inventor material library.
7. There is no better way to handle, please export the sheetmetal rule one by one and re-import to a clean sheetmetal xml one by one, sorry for that. please get the clean sheet metal xmls from enclosed file.
Hope it helps!
Xun