Oh I see OP. Yes that makes it more difficult.
I really think that the first name choice should be yours, then. Even if you marry, you aren't going to share a surname with this baby - yes you should have first choice of the first name. Because - you say you 'can't' give the baby your surname... well, you could, legally. Because it's your name. And your daughter's name. Your DP should be very relieved that you aren't saying that the baby WILL have your own family surname, that it's more important that it shares with its own sibling and mother... especially after your last experience. He's very lucky. He should quit while ahead.
If you wanted to get a court order to add your own birth surname to your older DD's, you'd probably get permission. If you wanted to go down that route, you could think of double-barrelling or having your birth surname as a middle name for both.