I would like to create heading numbering as follows:

i.1 Table of Contents (H1)
(counter increase)
1.1
1.2
(counter increase)
2.1
2.2
2.2.a
2.2.b
(counter increase)
3.1 conclusion

Right now I'm using heading/outline numbering with the style:
1 Heading 1 (dummy style, not used)
1.1 Heading 2
1.1.a Heading 3

Is there a way to incorporate some kind of counter into heading
numbering? Also, can I have the table of contents start with i.1? I
could do that manually if necessary.