Then you either demand dedicated "anti-office hours" or you tell them that it's not feasible that project B will get done with the way things have been recently. They're not technical, and all they can do is take your word on when you think you can finish it. They're not going to pick up on the caveats you've laced in your estimate. Also, you should let them know ahead of time that things are slipping instead of them finding it "behind schedule" on their own.
Honestly, don't take this as an insult, but it just doesn't sound like you're the right person for the position. If you have trouble pushing back when you need to, then you should find a spot where there is someone pushing back for you.
I agree with you that I'm not the right person. I was brought in to change the culture so we do everything faster(you can imagine the speed at a company with couple offshore devs and half dozen onshore people working on product).
I've, err we, have mostly failed.I can go on about why but at the end, I think we all share the blame and evaluate parts we could have personally done better and apply it next time round.
One of the big realizations for me is that they need someone older and more confident. I'm in my mid 20s and once founded a startup that got 30x the traffic but even months in, the boss would be unsure if I was a good programmer. We tried everything from emailing my code samples to his CTO friends to code reviews by other devs(all of which gave positive feedback) but I feel he could never totally shake that doubt. I called him out on it finally(basically if you can't tell after 8 months if I can program you should let me go). I've been programming since 12 but the lingering doubts actually started making me question my own abilities at which point I decided I need to move.
Honestly, don't take this as an insult, but it just doesn't sound like you're the right person for the position. If you have trouble pushing back when you need to, then you should find a spot where there is someone pushing back for you.