There is a strong distinction between "to intercede with Hashem", as opposed to "on behalf of Hashem".
The Commandment of Seeking the Dead is the mere Communication with the Dead (as a source of prophecy). We see this clearly by Shaul, after not getting Word of God from the proper channels, (prophets,אורים) he sought Shmuel (who was not alive) for guidance and understanding. (Shmuel 1, 28;6-15) (Devarim 18;11) (Shemot 20;2)
וַיִּשְׁאַ֤ל שָׁאוּל֙ בַּיהֹוָ֔ה וְלֹ֥א עָנָ֖הוּ יְהֹוָ֑ה גַּ֧ם בַּחֲלֹמ֛וֹת גַּ֥ם בָּאוּרִ֖ים גַּ֥ם בַּנְּבִיאִֽם׃
And when Saul inquired of the Lord, the Lord answered him not, neither by dreams, nor by Urim, nor by prophets.
We see it was the desperation of Shaul for not receiving Word from God which led him to seek out the Dead
וַיֹּ֨אמֶר שָׁא֜וּל לַעֲבָדָ֗יו בַּקְּשׁוּ־לִי֙ אֵ֣שֶׁת בַּעֲלַת־א֔וֹב וְאֵלְכָ֥ה אֵלֶ֖יהָ וְאֶדְרְשָׁה־בָּ֑הּ וַיֹּאמְר֤וּ עֲבָדָיו֙ אֵלָ֔יו הִנֵּ֛ה אֵ֥שֶׁת בַּעֲלַת־א֖וֹב בְּעֵ֥ין דּֽוֹר׃
Then said Saul unto his servants: 'Seek me a woman that divineth by a ghost, that I may go to her, and inquire of her.' And his servants said to him: 'Behold, there is a woman that divineth by a ghost at En-dor.'
*From This Passuk we see in fact אוב is the sin to be done by this Woman which is the prohibition of Seeking The Dead -
וְחֹבֵ֖ר חָ֑בֶר וְשֹׁאֵ֥ל אוֹב֙ וְיִדְּעֹנִ֔י וְדֹרֵ֖שׁ אֶל־הַמֵּתִֽים׃
or a charmer, or a consulter with a familiar spirit, or a wizard, or a necromancer.
וַיֹּ֤אמֶר שְׁמוּאֵל֙ אֶל־שָׁא֔וּל לָ֥מָּה הִרְגַּזְתַּ֖נִי לְהַעֲל֣וֹת אֹתִ֑י וַיֹּ֣אמֶר שָׁ֠א֠וּל צַר־לִ֨י מְאֹ֜ד וּפְלִשְׁתִּ֣ים׀ נִלְחָמִ֣ים בִּ֗י וֵאלֹהִ֞ים סָ֤ר מֵֽעָלַי֙ וְלֹא־עָנָ֣נִי ע֗וֹד גַּ֤ם בְּיַֽד־הַנְּבִיאִים֙ גַּם־בַּ֣חֲלֹמ֔וֹת וָאֶקְרָאֶ֣ה לְךָ֔ לְהוֹדִיעֵ֖נִי מָ֥ה אֶעֱשֶֽׂה׃
And Samuel said to Saul: 'Why hast thou disquieted me, to bring me up?' And Saul answered: 'I am sore distressed; for the Philistines make war against me, and God is departed from me, and answereth me no more, neither by prophets, nor by dreams; therefore I have called thee, that thou mayest make known unto me what I shall do.
From this Passuk we can deduce that the Seeking of Dead is for the Purpose of Guidance from above
The process mentioned above of "interceding with Hashem" would not fall under this command of seeking the Dead, rather it is Idol worship of someone that is Dead, no different than Stone and Wood
אָֽנֹכִ֖י֙ יְהֹוָ֣ה אֱלֹהֶ֑֔יךָ אֲשֶׁ֧ר הוֹצֵאתִ֛יךָ מֵאֶ֥רֶץ מִצְרַ֖יִם מִבֵּ֣֥ית עֲבָדִֽ֑ים לֹֽ֣א־יִהְיֶֽ֥ה־לְךָ֛֩ אֱלֹהִ֥֨ים אֲחֵרִ֖֜ים עַל־פָּנָֽ֗י׃
“I am Hashem your God, who brought you out of the land of Egypt, out of the house of bondage. “You shall have no other gods before me.
(Idol worship is even with Hashem)
To answer your final question, Although seeking the Dead is not included in the Noahide laws, serving and praying to any other than God, even with God would be considered Idol worship.
There is a distinction if the service to a specific Idol is not the "irregular practice of service" but, your question it seems like the normal practice of service.
Just to Point out as well, there are opinions who state that שיתוף (partnership) for a non-jew is permitted. Tosafot in Bechorot 2b, mentions putting a non-jew in a position to take a oath would be okay if he agrees to the one living God of Heaven and Earth and includes another "saint" as well. Some interpet Tosafot that a non-jew is permitted in all cases not just oaths but, even service.
In Your case, it is direct asking of the "saint" and including God in the service. This is different than Serving God with added "saints" (which is what Tosafot intended by שיתוף) and would be not permitted.
Remember that all Idol Worship is שיתוף, very few times in the Torah was there service to Idols without inclusion of God. (etc. Sun, Moon, Stars, Angels, Hosts) The question is which is the focus, if God is the focus than you can rely on Tosafot, if the other Gods are the focus, then even Tosafot would agree that it would be Idol Worship.
(In my experience with Christianity (which is not much), I see a-lot of variation with this and it seems that each person has his approach to this. Some you can hear the name of the "saint" more than God, and some you can hear God, and less the name of the "saint", my point is that it's not the religion but, the specific services in the religion)