I want my manager's job, with complications.

Hello to all of the smart and mature forum members--

I want my manager's job. Or half of it. 

I work in B2B account management on client team that historically has 2 people. The lead account manager has had 2 children recently and has spent almost all of the last 24 months on maternity leave or medical leave for a complicated pregnancy. For the second maternity leave, the account was put under another account manager in addition to his existing account instead of hiring in a dedicated manager. The team was reduced to 1.5 hc and we've grown the business with this customer by 36% at the same time.

The former account manager should be coming back in January, and I have heard it is not necessary that she will take this account back. The maternity cover has had a hard time handling both accounts-- he's slightly behind on issues, hasn't made many relationships within the client organization, hasn't built a account development plan or landed any new business.

I've been working with this client for a few years now -- and this last year I have tried to structure my activities and accomplishments around the 100%/50% recommendation from MT to prepared for a promotion. I've made a couple of solid wins for new business this year and have strong relationships with our key contacts in the client organization and our operations teams. I was recently nominated for our commercial development program aimed at 'accelerating mid-level commercial talent'. 

Is there any way to ask for the lead role on this account when the structure is reconsidered this winter?

I report into the maternity leave cover and I know he wants to stay on the account and keep his double role. Is it possible to position myself for this opening without implicitly undermining my manager?

Thanks very much for any and all advice!!