I was going through your second question but unclear about the default DCB that you are talking about. Please mention an example. To my knowledge you cant use a different LCREL to get the data, your job may fail.
If you are hesitating to approach the system support people to find the header information (as they ask many approvals etc..) as Dick mentioned, my suggestions to find the LRECL, (if you have not done the following)
1. Search for the dataset name (at least partial) in the PRODUCTION JCL library, PROC library, datacard library to check whether any job created that dataset (If it is created in the recent past then definitely you will find the job)
2. If you dont find the dataset name in the production then, as per the naming convention used in your TEST region, search for the dataset name using the prefix used in test region
3. If you dont find in both the above cases, then try for any similar kind of dataset name may be for different purposes, like backup or Inbound/Outbound files for different vendors etc.. with the approximate you may find the LRECL
4. If you dont find the dataset (at least partial) name in any of your dataset libraries, then only way to find is as mentioned by Dick
Hope this information helps..