It's correct that mood swings and depression aren't considered a part of ADHD but both are common comorbidities (along with ASD which can be misdiagnosed as either of them) - so yeah, a lot of us deal with a whole gift basket worth of different issues. Personally, I've struggled with depression and am trying to get a formal evaluation for ASD and I suspect you'll find depression is pretty common in this community as it's a comorbidity I've seen extremely often in ADHD folks I know IRL.
It's important to make sure your doctor is aware of the different things you're experiencing and dealing with them separately - stimulant medications can mask depression symptoms until you fall off a cliff.
In terms of plans going awry, if you can, make sure your friends and acquaintances are aware of your ADHD and, especially, that sometimes you'll be flakey. If your network accepts that it'll be easier for you to flake and easier for you to recover. In my youth I was aware of how flakey I could be but flaking out on something would fill me with so much guilt that I'd end up taking much longer to recover.
Being honest with yourself and accepting the time lost to strong emotions is always going to be more healthy than struggling to try and stay "normal".