If the MAX® 10's JTAGEN pin is enabled and switched High / Low in user mode, can it be switched to User I/O or JTAG pins?

Yes, it is possible. 
See also the Knowledge Database below.

Why can't I access the MAX 10 device via JTAG even though JTAGEN is pulled high in user mode? 
 

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Category: Programming/Configuration 
Tools: Quartus® Prime 
Devices: MAX® 10