While tevilah usually takes place after nightfall, in a case of need it is permissible to immerse right after sunset. If you can make arrangements with the mikveh attendant to immerse shortly after sunset, you can have your husband drive you to the mikveh beforehand to save yourself the extra walk getting there, and then you will have enough time to walk back afterwards in time for the seder. You can even make an excuse to your family that you are going to shul or visiting a friend, etc.
If this scenario doesn't work out, it is permissible to tell your mother (or sister, etc.) that you need to go to mikveh, so she can help cover for you while you are late. Often, until shul ends and everyone gets home and finishes getting the seder table ready, it can be more than a half an hour after nightfall, so you may not be as late as you suspect.
We hope these suggestions help you manage the logistics of immersing on seder night. Ultimately, if it doesn't work out, and you and your husband agree, you may delay immersion until after Yom Tov.
Please feel free to get back to us with any further questions.
Chag Sameach!