You have quite a list of questions!
1. You must upgrade your NLM to the version provided on your 2010 media.
2. The FlexLM is backwards compatible and should not affect your existing products. The latest version still has the same look and feel that you are used to.
3. You only need the INCREMENT and PACKAGE portions of your 2010 license. You do not need any of the header information. Also, your LIC file should only contain one PLIST entry from your latest license file.
4. No. You do not have to leave blank lines, but it does help for clarity when you have multiple licenses. This is personal preference.
5. No, there really isn't anything more to be concerned about with your LIC file. Just append your 2010 to the end and resave.
6. To combine similar products (i.e. two sets of 2010) you need to have both licenses created on the same date. When they are created on the same date, the NLM will combine the amount of licenses appropriately. Otherwise, the file with the latest creation date will take priority. Also, if your 2010 is under Subscription and has a PACKAGE line, then it is backwards compatible for licenses going back to 2009, 2008, and 2007 and will subsequently supercede any other existing licenses of those types in your LIC file. If you do not want this, have Autodesk generate your LIC files without the PACKAGE option.
7. Follow the above information and your 2010 licenses should work. You will need to update your NLM and properly append your 2010 licenses for it to work. Your status enquiry in LMTOOLS should provide you with the information showing that your licenses are available, when configured properly.
If you have any additional questions, simply post them here.
Good Luck!