When you select your almost-MaxSpace setting, the engine for backup becomes WIMLIB (it has to - only it provides compression settings above MaxSpeed), and the restore engine remains at the default of WIMGAPI.
It is surprising to hear that you saved more space with WIMGAPI compared to WIMLIB; this is I am certain a result of an uncontrolled test environment - WIMLIB's compression far exceeds that of WIMGAPI.
In the long term, we'll be ditching WIMGAPI completely as the open source WIMLIB library matures more and more. The developer has, in fact, added several fixes and workarounds to his library - that should aid in the prevention of the types of errors you encountered (or, at least, making them non-fatal - such that, any non-fatal errors would not abort the operation and end up breaking your PC).
So anyways, I'm glad that it finally worked for you - stay tuned for more updates in the future. We've already added a fifth compression grade between MaxSpace and MaxSpeed; and are working to integrate further fixes from the WIMLIB author even as I type this - this should take care of the access problems, as well as the extraction problems that have been previously reported - hopefully, letting us permanently ditch the WIMGAPI engine moving forward. There'll be a new update as soon as we can deliver it