Change from 240V to 120V
First, congratulations.  This is the most well-thought-through presentation of a plan that ai have ever seen on this forum, and your thoroughness has paid off - you got everything right, including an ambitious use of a MWBC, with all its details right.  

By your first paragraph I was thinking "MWBC" and voilà you're already there.  Well done. 

The breaker

Is absolutely fine and I would not mess with it.  Even if you were to downgrade to a simple 120V circuit, it is fine to just use half of the 2-pole breaker and disconnect the other wire from the breaker and cap it in the panel.   

Your idea to go with a single breaker isn't wrong but it will leave a gaping hole in your panel.  They make blanking covers but they're hard to find, expensive, and flimsy --  so I prefer to fill empty holes with actual breakers and just mark them n/c.  So then you'd be right back where you started, but $10 poorer and with a nice 2-pole breaker to mislay. 

The only breaker swap I would consider is swapping to a 2-pole GFCI breaker.  That lets you provide the GFCI protection for both half-circuits in a dry location, so you aren't paying extra for outdoor GFCIs or enduring their horrible failure rates.  The 2-pole GFCI breaker would also allow you to use every MWBC trick in the book without having to worry about how GFCIs interact with MWBCs - from hot-skipping on a long line of receptacles, to provisioning a 240V receptacle also.  

How do you de-energize the panel to access the breaker?

If there's a big main breaker, that will do it -- however if this breaker sits in a zone of 4-6 breakers marked "Main Breaker", then you cannot de-energize that section.  Fortunately you don't have to - modern breakers like these "tip and snap in".   You can snap the breaker out, wires still attached, and hold it in your hand while you change the wires.