When Issur and Tumah Collide

The Minchas Chinuch in Mitzah 263 (Din Tumas Kohanim) writes as follows:

And behold I will not refrain from writing what I don't know even though it is possible that it is a davar pashut for one who is a baki in shas and he has good sechel. Still a man like me is not embarrassed to write what is in doubt to me. Behold it is well known in all of shas that safek tumah in Reshus HaYachid is tamei... and safek tumah in reshus harabbim is tahor... and this matter is only by tumah but by issurin there is no nafka minah between reshus hayachid and reshus harabbim... and behold if one is tamei with a safek tumah in reshus hayachik he is forsure tamei... I am in doubt what is his halacha as far as issur for example like over here with a kohen and nazir that if there is a safek tumah in reshus hayachid or even sfek sfeka if he touches it he is forsure tamei... But if they warned him not to become tamei and he transgressed and became tamei does he get malkos? Or do we say that the Torah revealed this only for tumah...

In other words, the Minchas Chinuch is mesupak as to what is the halacha of safek tumah in reshus hayachid when it comes to the issur of coming into contact with tumah. Do we apply the rules of safek tumah or the rules of safek issur?

The Esvan D'Orayssa in Klal 21 deals with a similar issue. He introduces the piece as follows:

Here we will speak on the inyan of niddah to her husband. Is it issur or tumah?

At the end of the piece he quotes an Aderes Eliyahu from the Gra as follows:

And see also the Gra in Sefer Aderes Eliyahu where he says as follows in Parshas Shoftim on the pasuk of Ki Yipalei Davar etc. and here are his words:

That the torah is divided into six parts tahor and tamei, assur and mutar, and chavav and zakai. Bein Dam L'Dam this is Issur and Mutar to make a woman pure for her husband...

The Esvan D'Orayssa finishes with the following words:

And it is clear that dam niddah and zivah that assur a woman to her husband are in the category of issur and not in the category of tumah.

* For more on this topic see Siman 2 of Sefer Eretz HaTzvi by Rabbi Hershel Schachter.