Please go through the README.txt provided in the SDK it cover all the supported features and known limitations. You might find it helpful.
There are no references in the README.TXT file that address these questions, let alone the fact that the WiFi setup function simply locks up with short PASSPHRASE when I would expect it to report an error !
Are there any restrictions on string length (minimum and maximum)?
The SSID can have a maximum of 30 chars:-
This is what I tested it with: %L:~t}}FTjn3?6Bq+QZ42(ftUwfR/T
Answers your second question as well.
Are there restrictions on the character code values? ASCII 0x20 to 0x7E have been suggested.
The AP_SECURITY in the default_wifi_config_dct.h is set to WPA2_MIXED_PSK. You can use 0x20 to 0x7E as long as the length of the pass phrase remains between 8 and 63 chars.
Are there any restrictions on character mix? (e.g. mixed upper and lower case, minimum of upper and lower case characters, minimum numeric count)
No restrictions on the number of characters to be mixed.