Have you tried the Mms.apk from the latest CAI? I went with option C this time, before it was option D. They removed that checkbox back in March for split sms.
a) (default) enableMultipartSMS is true, enableSplitSMS is false.
The application will always send one large sms to the radio, which will split at hardware level.
b) enableMultipartSMS is false, enableSplitSMS is false.
The application will send one mms if we reach the smsToMmsThreshold, else one large sms is sent.
c) enableMultipartSMS is true, enableSplitSMS is true.
The application will always split the sms before sending to the radio.
d) enableMultipartSMS is false, enableSplitSMS is true.
The application will send one mms if we reach the smsToMmsThreshold, else it splits the sms before sending.