Display Only a Subset of a Menu

I am working on a site, and I’ve got a top menu with dropdowns. Those are working perfectly. They are using the built-in menu functionality in Appearance>Menus.

Now I need to build out a left nav that pulls from the same menu (called MainMenu).

If my full menu is this:

Top Level 1
-Second Level 1
-Second Level 2
–Third Level 1
–Third Level 2
-Second Level 3
-Second Level 4
–Third Level 1
–Third Level 2
–Third Level 3
Top Level 2
-Second Level 1
-Second Level 2
Top Level 3

If I was on any page under Top Level 1, I need to display all pages in Top Level 1, so it should display:

Top Level 1
-Second Level 1
-Second Level 2
–Third Level 1
–Third Level 2
-Second Level 3
-Second Level 4
–Third Level 1
–Third Level 2
–Third Level 3

I also need to add something to any Second Level pages that has children. I need to add a SPAN tag after the closing a tag for Second Level 2 and Second level 4. I’m doing this to create an accordion menu in JQUERY. The spans will hold an arrow graphic that, when clicked, reveals the sub-pages underneath. I cannot just CSS the arrow as a background on the pages with children, because those links go to pages.

Now I know that a Walker Class is most likely needed, but I don’t know a thing about them. I’ve copied and pasted examples before, but I honestly have no idea what a lot of the code is doing, I just know it works.