All Sage Mode users mix NE with their chakra to form Senjutsu chakra, so all their attacks are Senjutsu attacks. In other words all Sage Mode users are Senjutsu users. That's not so say all Senjutsu users are Sage Mode users.
Right, I see that I may have oversimplified things to make my point, so I'll clarify here. I agree that using senjutsu doesn't automatically make you a Sage Mode user; rather, it would be more accurate to say that mixing ambient natural energy and chakra is what produces senjutsu chakra, which Sage Mode utilizes. When I say "Six Paths Senjutsu," what I really mean is "Six Paths Senjutsu
Chakra."
I appreciate how you referred to Madara's use of senjutsu as "a loophole," but to be more specific (as this is an important point), here's how I'd explain it: Senjutsu users that don't use Sage Mode do undergo a similar process to the real thing, the difference being that they tend to employ a middle-man to transfer the chakra as opposed to tuning themselves with nature to collect it directly. In the case of Orochimaru's curse marks, that middle man is Jugo's bloodline limit, which passively absorbs and stabilizes ambient natural energy. Likewise, in the case of Madara, the Ten Tails itself is that middle man, having absorbed vast quantities of nature energy as the Shinju.
What's important to remember here is that because of the different method of acquisition, or because of the nature of the middle man's role, Orochimaru's curse marks come with unique advantages and disadvantages compared to Sage Mode. I contend that the case of the Ten Tails' Jinchuriki follows this same trend--more on that below.
So Six Path Senjutsu is different from Six Path Sage Mode only in that, Six Path Sage Mode is a mode and Six Path Senjutsu is what the mode can do.
See, that's where I have to disagree. As I pointed out in my original post, Six Paths Senjutsu has unique properties that normal Senjutsu does not; most notably, as stated in the 4th databook, Truth Seeker's Orbs appear when Six Paths Senjutsu "blossoms" within an individual. But they didn't appear when Naruto activated Six Paths Sage Mode in chapter 672, and the Six Paths Sage Mode databook page makes no mention of them. Instead, Truth Seeker's Orbs manifested specifically when Obito and Madara became the Ten Tails' Jinchuriki, when Kaguya stabilized her Ten Tails transformation, and when Naruto activated his Tailed Beast chakra cloak.
This leads me to believe that the databook's (and possibly Madara's) use of the term "Six Paths Senjutsu" is specifically referring to the senjutsu chakra of the Ten Tails. Taking into account that the power of the Ten Tails has several times been referred to as the power of the Sage of the Six Paths by Madara, Sakura, and others, I don't think that's too much of a stretch.
SPS is a power that Madara and Naruto both use, and comes from the Ten Tails. SPSM is mode that Naruto and Ashura both use, and comes from Hagoromo himself. What I'm arguing is not that one of them isn't senjutsu, but rather that they're both different kinds of senjutsu that have a similar name because they both originate from the Sage of Six Paths.
But that's not to say he has lost the bijuu chakra, there's absolutely no evidence of that.
Isn't there? Refer back to my original post; Sasuke clearly states at the Valley of the End that Naruto is running low on Tailed Beast chakra, and unlike Kurama, the other Tailed Beasts themselves aren't actually sealed within Naruto. This is confirmed by Hagromo after Kaguya's defeat. He only possesses gifted chakra, which in has been shown in other cases (such as Naruto's gifting of Kurama's chakra to the Alliance) to not recover over time.
