try changing the no option:
<input <?php if (!(strcmp($row_WADAlistings['MobilityIcon'],"no"))) {echo "checked=\"checked\"";} ?> type="radio" name="ckmobility" id="radio2" value="no" />
value to blank:
<input <?php if (!(strcmp($row_WADAlistings['MobilityIcon'],"no"))) {echo "checked=\"checked\"";} ?> type="radio" name="ckmobility" id="radio2" value="" />
Then in the Update record server behavior, set the data type for the checkbox column to be Checkbox Yes/ No