Corruption of ROM BIOS is a real possibility as documented on page 58 of the DOS Compatibility Card User's Guide. If the DOS Compatibility Card only has 2 MB of RAM and the TESTMEM switch is not off, the card is unusable because the ROM BIOS becomes corrupted during the memory test.
The minimum memory requirements for the 6100 DOS Compatibility Card are at least 8 MB RAM with 2 MB shared for the PC. This is documented on page 90 of DOS Compatibility Card User's Guide. The requirements listed on the box for the DOS Compatibility Card are 4 MB for the card plus the memory required for the Macintosh OS. Since System 7.5 requires 8 MB on the Power Macintosh computers, there should be a minimum of 12 MB of RAM in a Power Macintosh using System 7.5. The memory requirements are lower when System 7.1.2 is used because 7.1.2 only require 4 MB RAM.
The F5 and F8 keys do work on the DOS Compatibility Card. You have to wait for the "Starting MS-DOS..." message to appear before you press the function keys. If the keys are pressed before this message is displayed, they are ignored by MS-DOS. Other MS-DOS computers also have this same behavior.