[Droid Incredible Q&A, Help & Troubleshooting] DavidPhx:
After issueing that mkdir command like you suggested, I tried again where I left off in step 4, and was able to proceed with the rest of the instructions in step 4 and get Froyo loaded and running. Making good progress here, thanks Tiny!

The writing of the PB31IMG.zip file:

C:\Users\David\Downloads\ADB-Tools>fastboot flash zip PB31IMG.zip
sending 'zip' (176625 KB)...
OKAY [ 25.167s]
writing 'zip'...
(bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) checking main version...
(bootloader) start image[hboot] unzipping for pre-update check...
(bootloader) start image[hboot] unzipping & flushing...
(bootloader) [RUU]UZ,hboot,0
(bootloader) [RUU]UZ,hboot,100
(bootloader) start image[radio] unzipping & flushing...
(bootloader) [RUU]UZ,radio,0
(bootloader) [RUU]UZ,radio,9
(bootloader) [RUU]UZ,radio,18
(bootloader) [RUU]UZ,radio,23
(bootloader) [RUU]UZ,radio,32
(bootloader) [RUU]UZ,radio,41
(bootloader) [RUU]UZ,radio,46
(bootloader) [RUU]UZ,radio,51
(bootloader) [RUU]UZ,radio,60
(bootloader) [RUU]UZ,radio,69
(bootloader) [RUU]UZ,radio,78
(bootloader) [RUU]UZ,radio,87
(bootloader) [RUU]UZ,radio,97
(bootloader) [RUU]UZ,radio,100
(bootloader) [RUU]WP,radio,0
(bootloader) [RUU]WP,radio,7
(bootloader) [RUU]WP,radio,16
(bootloader) [RUU]WP,radio,21
(bootloader) [RUU]WP,radio,26
(bootloader) [RUU]WP,radio,35
(bootloader) [RUU]WP,radio,40
(bootloader) [RUU]WP,radio,45
(bootloader) [RUU]WP,radio,50
(bootloader) [RUU]WP,radio,100
(bootloader) start image[boot] unzipping & flushing...
(bootloader) [RUU]UZ,boot,0
(bootloader) [RUU]UZ,boot,43
(bootloader) [RUU]UZ,boot,83
(bootloader) [RUU]UZ,boot,100
(bootloader) [RUU]WP,boot,0
(bootloader) [RUU]WP,boot,42
(bootloader) [RUU]WP,boot,85
(bootloader) [RUU]WP,boot,100
(bootloader) start image[recovery] unzipping & flushing...
(bootloader) [RUU]UZ,recovery,0
(bootloader) [RUU]UZ,recovery,20
(bootloader) [RUU]UZ,recovery,43
(bootloader) [RUU]UZ,recovery,73
(bootloader) [RUU]UZ,recovery,99
(bootloader) [RUU]UZ,recovery,100
(bootloader) [RUU]WP,recovery,0
(bootloader) [RUU]WP,recovery,20
(bootloader) [RUU]WP,recovery,40
(bootloader) [RUU]WP,recovery,60
(bootloader) [RUU]WP,recovery,80
(bootloader) [RUU]WP,recovery,100
(bootloader) start image[system] unzipping & flushing...
(bootloader) [RUU]UZ,system,0
(bootloader) [RUU]UZ,system,6
(bootloader) [RUU]UZ,system,12
(bootloader) [RUU]UZ,system,19
(bootloader) [RUU]UZ,system,25
(bootloader) [RUU]UZ,system,32
(bootloader) [RUU]UZ,system,39
(bootloader) [RUU]UZ,system,45
(bootloader) [RUU]UZ,system,51
(bootloader) [RUU]UZ,system,58
(bootloader) [RUU]UZ,system,64
(bootloader) [RUU]UZ,system,71
(bootloader) [RUU]UZ,system,78
(bootloader) [RUU]UZ,system,84
(bootloader) [RUU]UZ,system,89
(bootloader) [RUU]WP,system,0
(bootloader) [RUU]WP,system,89
(bootloader) [RUU]UZ,system,89
(bootloader) [RUU]UZ,system,89
(bootloader) [RUU]UZ,system,90
(bootloader) [RUU]UZ,system,91
(bootloader) [RUU]UZ,system,92
(bootloader) [RUU]UZ,system,93
(bootloader) [RUU]UZ,system,94
(bootloader) [RUU]UZ,system,94
(bootloader) [RUU]UZ,system,96
(bootloader) [RUU]UZ,system,96
(bootloader) [RUU]UZ,system,97
(bootloader) [RUU]UZ,system,98
(bootloader) [RUU]UZ,system,99
(bootloader) [RUU]UZ,system,100
(bootloader) [RUU]WP,system,89
(bootloader) [RUU]WP,system,89
(bootloader) [RUU]WP,system,90
(bootloader) [RUU]WP,system,91
(bootloader) [RUU]WP,system,92
(bootloader) [RUU]WP,system,93
(bootloader) [RUU]WP,system,94
(bootloader) [RUU]WP,system,94
(bootloader) [RUU]WP,system,95
(bootloader) [RUU]WP,system,96
(bootloader) [RUU]WP,system,97
(bootloader) [RUU]WP,system,98
(bootloader) [RUU]WP,system,99
(bootloader) [RUU]WP,system,100
(bootloader) start image[sp1] unzipping & flushing...
(bootloader) [RUU]UZ,sp1,0
(bootloader) [RUU]UZ,sp1,100
(bootloader) [RUU]WP,sp1,0
(bootloader) [RUU]WP,sp1,100
OKAY [153.388s]
finished. total time: 178.557s

C:\Users\David\Downloads\ADB-Tools>
C:\Users\David\Downloads\ADB-Tools>fastboot reboot
rebooting...

finished. total time: 0.097s

C:\Users\David\Downloads\ADB-Tools>