Notice נציר in Samaritan = 'devoted.' Another argument that you can make is that the Samaritan situation demonstrates that the 'switching' from nṣr to nṭr or the mistaking of one for the other was common place. In other words, they must have sounded the same or close enough that the meaning of originally separate words were take to be related or even interchangeable. The situation in Samaritan Aramaic reflects the confusion that occurs in early Christianity and in specific Adv Marc. The author is likely not a Greek speaking 'ignoramus' who mistook nṣr for nṭr but reflecting contemporary usage of native speakers.
Note the reflection of this situation at the end of Adv Marc 4:23:
When, however, He answers the man, who alleged as an excuse his father's burial, "Let the dead bury their dead, but go thou and preach the kingdom of God," He gave a clear confirmation to those two laws of the Creator----that in Leviticus, which concerns the sacerdotal office, and forbids the priests to be present at the funerals even of their parents."The priest," says He, "shall not enter where there is any dead person; and for his father he shall not be defiled" ; as well as that in Numbers, which relates to the (Nazarite) devotion; for there he who devotes himself to God, among other things, is bidden "not to come at any dead body," not even of his father, or his mother, or his brother. Now it was, I suppose, for the (Nazirite) devotion and the priestly office that He intended this man whom He had been inspiring to preach the kingdom of God. Or else, if it be not so, he must be pronounced impious enough who, without the intervention of any precept of the law, commanded that burials of parents should be neglected by their sons. When, indeed, in the third case before us, (Christ) forbids the man "to look back" who wanted first "to bid his family farewell," He only follows out the rule of the Creator. For this (retrospection) He had been against their making, whom He had rescued out of Sodom.
Illi autem causato patris sepulturam cum respondet, Sine mortui sepeliant mortuos suos, tu autem vade et annuntia regnum dei, utramque legem creatoris manifeste confirmavit, et de sacerdotio in Levitico prohibentem sacerdotes supremis etiam parentum interesse (Super omnem, inquit, animam defunctam sacerdos non introibit, et super patrem suum non contaminabitur), et de devotione in Arithmis; nam et illic qui se deo voverit inter cetera iubet ne super ullam animam introeat defunctam, ne super patris quidem aut matris aut fratris. Puto autem et devotioni et sacerdotio destinabat quem praedicando regno dei imbuerat. Aut si non ita est, satis impius pronuntiandus qui nulla ratione legis intercedente sepulturas parentum despici a filiis imperabat. Cum vero et tertium illum prius suis valedicere parantem prohibet retro respectare, sectam creatoris exequitur. Hoc et ille noluerat fecisse quos ex Sodomis liberarat.
It would seem then the author knows that Nazirite means 'devotion' but also - and this is important - there is an added implication that a 'Nazirite' means Christian 'priest.' Consider also that among the Mandaeans Nazarene is a title which is reserved for the priests.
I don't know if I am making this clear enough but it would seem
(1) that Nazarene is assumed to mean Christian (see the statement in Adv Marc 4.7)
but then
(2) Nazarene is assumed to have the meaning of Nazirite as we see in Samaritan Aramaic.
and finally
(3) a 'full' Christian is necessarily a priest (something which is reflected in the early practice of the baptism of the catechumen, the Letter to Theodore from Clement of Alexandria). In other words, anyone can believe in Jesus but baptism is reserved for those who have been prepared for a special devotion to God and receive baptism coming over from the catechumen.