Opinion dose not count against facts and the facts of the Manga dose not support your believes. Over the years some fan faction writhers have suggested that Minato is the son of Tsunade and Dan or Tsunade and Jiraiya and Kishi has said no to such speculation just because you can used a jutsu dose not mean you are related. Sakura can use some of Tsunade’s jutsu dose that mean She is Tsunade’s granddaughter? No, it dose not.
And what manga facts are these that you speak of? Coz last time I checked, the manga neither confirmed nor denied their relation to each other, so please, tell me what are these irrefutable facts that you speak of?
And while we're at it, let me give you some manga FACTS that support the OPINION that Minato might be a senju/Tobirama relative.
1)
The obvious one is the fact that they both use FTG, especially when you consider that such techniques are normally past on to relatives, kinda like how Jiraiya passed on rasengan to Naruto because he is Minato's son, despite Naruto not knowing that he is in fact related to Minato. This is a possible and likely scenario that Minato was given access to this jutsu because of family relation, otherwise how do you explain why it was passed on to him specifically and no-one else?
2)
Physical build, appearance, hair style, and personality/intellectual/way of thinking. Character traits that can be passed on through genetics.
3)
Chakra levels. Minato displayed chakra levels unheard of from supposedly non-senju/Uzumaki people. FTG required three elite jonin (Minato's bodyguards) to teleport one person and yet Minato and Tobirama teleport themselves and others without any effort whatsoever. That's insane chakra right there, not to mention how many times Minato actually teleports and the size of the things he teleports (bijuu dama and kyubi).
4)
Durability - Minato took a claw through the stomach from Kyubi like boss, stopped Kyubi's movements, didn't flinch and went on to execute high level jutsu after the fact, enough said.
5)
Kyubi hosting - it was explicitly stated that only Uzumaki/Senju can host the kyubi, Minato not only hosted it but became a PERFECT jinchuuriki. That's Senju level no matter how you wana look it. In case you're in denial. Before you go the whole 'edo' excuse, that's not valid, Madara explicitly stated that edo bodies are weaker than normal bodies, and he even said he needed his human body to become a jinchuuriki. Tobirama also stated that edos are weaker than normal bodies. This means that Minato was able to control kyubi already when he was still alive, and if he mastered it as an edo, then that's an even greater achievement.
6)
Sage Mode - Minato, it was stated by Pa that you need incredible chakra to even learn sage mode (this supports the senju chakra levels) but Minato took it further and mastered sage mode and became a perfect sage like his Uzumaki son and unlike Jiraiya. So far only people with Senju dna have mastered sage mode without help. Kabuto even directly stated that he needed to implant Karin's dna (Uzumaki/Senju) in order to master sage mode (Jugo's was needed to gather it passively at all times, not master it, in fact Jugo himself aint mastered it).
7)
Kyubi said that the seal that Naruto used to seal him was the sage of six path's seal, the fact that Minato has the same sage-like jinchuuriki mode suggests he used the exact same seal. That's a direct link to the sop, especially when u consider everything kyubi related Naruto got was plotted out by Minato.
8)
Children of prophesy - the are only three children of prophesy, and two of them are confirmed senju descendends, the other one, Minato, has HEAVY senju forshadowing (being a prophesy child being forshadowing in itself considering that the prophesy concerns the descendents of the sage) of being a senju.
9)
Tobi said that all the Hokage are related/linked to the Senju, and out of the seven of them, four are confirmed senju descendends.
10)
Uzumaki clan had a marriage treaty alliance with the Senju in which Mito married Hashirama, how Ironic that Minato, a Hokage linked to the Senju, like Hashirama, married the last known living Uzumaki at the time.
11) Chakra sensing:
Both displayed ridiculous chakra sensing feats of equal magnitude, that being sensing Madara and Naruto and even identifying particular details about their chakra from literally countries away as they were still in Konoha. This is an even greater feat when you consider that the last time Tobirama would have sensed Madara is BEFORE the Senju implants. It was clearly revealed as a manga fact that a person's chakra signature changes due to chakra implants. Minato showed a similar feat when he was able to sense that Madara's SOP form was stronger than Obito's, especially considering that Naruto in sage mode had said that the juubi's chakra was immeasurable/Kurama said so too.
More importantly regarding this category is the style/method/technique they both used to track a number of enemies during their respective wars, that being of placing the finger on the earth and identifying the number of enemies and their strength. This is a particular technique that only the two of them have been shown using.
Conclusion:
EVERYTHING about Minato SUGGESTS or FORSHADOWS a Senju/Tobirama lineage. Of course, it is not CONFIRMED by the manga, I agree with you on that. But if you believe it is not likely, or it is not IMPLIED or FORSHADOWED, then you are batshit crazy and should be checked into a mental asylum asap.