There are four nested questions. I do not addressed here the issue of the mitsva to fruitful and multiply:
1. Is there a mitsva for a woman to go to the mikveh when she is prohibited to her husband because of uncleanness or to get out of the problem of conjugal prohibition?
2. If the answer is yes, is there a mitsva to go to the mikveh immediately after the shiv'a nekiim, even independently of the presence of the husband, or independent of the opportunity to make the mitsva?
3. Is there a mitsva of marital relatioship at the night of the mikveh?
4. If the answer is yes We know a mitsva called mitsvat ona, a duty of the husband towards his wife, to ensure periodic marital relationships. Is the leil tevila, a greater mitsva? E. g. is leil tevila a duty even when mitsvst ona is skipped e. g. avelut or tish'a beav, to make the mitsva at the night of mikveh remains a duty? In other words is this mitsva greater than mitsvat ona?
a. For the first question Rashi in Betsa 18b understood from the Gemara that there is a mitsva, and not all prohibitions can postpone the mitsva. Chatam Sofer argued that nowadays the Tevila is already postponed because of the stringencies linked to Safek Zava and we cannot avoid to postpone the Tevila even because of rabbinical prohibitions. Bet Yosef follows the rule of the Gemara for several cases even nowadays. See further explanation of the Bet Yosef.
b. For the second question which is addressed in Gemara Nidda 31a, the conclusion of the Gemara according to Rishonim is that there's a such mitsva when we know exactly the right time of Tevila. But some Rishonim, e. g. argued that nowadays the right time is an uncertainty. So, according to them, no mitsva of "Tevila at right time". But the Bet Yosef (YD 197) argued that even nowadays we have a right time following the last halachic rules, the time is clearly after 7 clean fays, even when the woman did seen only a little drop of blood. Magen Avraham (OC 574, sk 5), Sidre Tahara (YD 184 sk 14) and Chatam Sofer (Shut YD 170 and 194) disagreed to the Bet Yosef view.
c. For the third question Rashi Berachot 24a learned from the Gemara that yes. Leil tevila is leil tashmish.
d. For the fourth question, Rashi understood from the Gemara in Berachot 24b that there is a special mitsva of relationship at Tevila night. Acharonim discussed if this duty is stronger than the mitsva of Ona. Sidre Tahara and Chatam Sofer said that it's not stronger and Bet Yosef did seen it as stronger than ona, as night before travel is. Chatam Sofer and ST argued even against the status of night before travel.
In conclusion LeHalacha velo Lemaasse. Following CS (reported in pitche teshuva YD 197) and SD, the Tevila can be delayed and according to the Bet Yosef no Tevila and no mitsva can be delayed.
See THE COMPREHENSIVE VERSION OF THE ANSWER for details