I think you need to go back to your boss and ask for confirmation of your hours, at this stage it's not relevant what negotiations your boss is / isn't having with your colleague. If you really get nowhere, I'd go with "if I don't hear from you by 5pm on X date, I'll presume I'm working my usual 2.5 days" then at least you can make your own arrangements for childcare etc and not be messed around.
I'm due to go back after maternity leave in late June. I work 2 1/2 days a week but my colleague who is returning the same time as me asked to drop her hours & I was asked to take them so would be working 3 days a week. I had no problem with this as long as I could get childcare so said yes subject to nursery being able to take the boys.
Nursery were fine so I confirmed to my boss that I could do the 3 days. He said he'd get back to me that week once he'd finalised the work pattern.
Cue 3 weeks later & I still haven't had confirmation from my boss. I have heard from my colleague that our boss isn't happy about the hours she wants to do so she may not be dropping her hours after all, which puts me back at square one.
Is it really acceptable that I'm left hanging with only 6 weeks of mat leave left?