I don't think it should be that difficult. However, it is important that in doing this you not use the buttons on the ribbon, but rather follow the steps shown at
How to create numbered headings or outline numbering in Word 2007 and Word 2010.
Note that you will have very confusing numbering if any of your numbered levels have more than nine items. Actually, you will have confusing numbering even without that confusion.
To clarify, I have added labels to your levels. Do I have it right?
1000 - level 1
1100 - level 2
(a) - level 3
(b) - level 3
1110 -level 4
(a) - level 5
(b) - level 5
1120 - level 4
(a) - level 5
1200 - level 2
2000 - level 1
Attached is a docx file with the numbering shown above. Note that you can have multiple instances of the same level four numbers.