Allow me to counter everything youve said. Firstly well done for telling us what we already know even though youve overestimated the combination of sm + km way too much, but also youve only looked at naruto's possible abilities whilst ignoring sasuke's. Maybe because you are making an argument for naruto or maybe we havent fully seen what ems sasuke can do.
Anyway jiraiya already had trouble controlling his chakra whilst in sm thats why we saw his part transformation when he was in sm. Secondly we can argue that the reason naruto was able to withstand nagato's chakra in sm + km was because nagato was already extremely low on chakra, evident in the use of gedo rinne tensei which effectively killed him.
Secondly km modes combat abilities is extremely similar to sm and infact both side abilities is also similar. Km allows the user to extreme speed, durability, strength and almost infinate chakra. Sm gives the user the name thing, but in this case natural energy. The side ability of sm is to sense chakra, whilst the side ability of km is to sense evil thoughts. If naruto was to combine both, he would only increase what both already gives him, unless kishi full something out of his ass off course.
You, say and i qoute; 'take chakra sensing, malice sensing, and sm's threat perception and add them together. which gives you a very early warning of opponents and attacks, people's intentions etc. ie it will be insanely hard to hit, naruto in sm has never even gotten hit let alone in both modes.'
Firstly being a chakra sensor isnt really a combat feat but rather a tracking feat. Thats isnt really useful in combat. You say malice sensing, well its a fight he doesnt need this ability to know that his opponent wants to kill him. He would also be on his guard regardless of this ability. None of this gives him an early advantage against opponents attacks as we saw against nagato and tobi, unless you are basically trying to say that he will get the same ability as spiderman, which he hasnt demonstrated. But also this ability doesnt leave even spiderman immune to attacks.
Sage mode katas is unique to sage mode. I do not believe that the combination of sm and km would allow for the use of sage mode katas because that combination isnt (sm+km) sage mode anymore. Lets say its possible, it is bound to take great concentration and probably another training arc.
Also you exaggerate about naruto having the sage's body. Firstly sm+ km doesnt give you the sage's body. The sage was able to not only surpress the ten tails chakra which is made up of 1-9 demons but also contain it. Naruto only has 1 of these demon chakra in him and even that its half the fox's chakra.
You say naruto was able to withstand damage from falling from huge hights therefore his body is basically 'invisble'. Although i agree that he did withstand damage from falling from huge hights, that doesnt mean ne is invisible. People seem to forget that naruto in km+sm was stabbed by nagato's chakra receivers. Also jiraiya in sm got not only his left arm taken off against pein, he got his throat shattered, chest stabbed and eventually stabbed in the back numerous times which in the end killed. These are all physical attack.
You do not want me to get into other types of attacks that would leave sm + km defenseless. What ive said is basically countering your argument for naruto and i havent even talked about sasuke's abilities
i didn't mention sasuke's abilities because the post i was replying to was saying that combining sm and km would be nearly useless. on the contrary, i think the combination is key to fighting on a par with sasuke.
both have weaknesses against each other tbh. naruto has been shown to struggle against genjutsu, and has no jutsu to piece a final susanoo as yet (maybe he can pull him out with chakra arms like gaara with sand, but that's an argument for another day). sasuke, on the other hand, has lower stamina even vs base naruto, and him struggling against speedy opponents is a recurring theme.
now, to your arguments. the bit about the chakra sensing was for me to make the point that naruto can track opponents very effective, and know the enemy's position, so he can't be caught off guard or sneaked upon, or hidden from. the threat perception gives him early warning of attacks, without blind spots.
nagato said he could easily control naruto at that range, so he definitely didn't expect to be overcome. he stabbed naruto in sm, not in sm+km. naruto awakened the combined mode later. and jiraiya had great chakra control, as evidenced by the fact he could perform a rasengan with one hand. him not having a perfect sm is irrelevant.
about the frog kata, could be, but since he mastered both modes i don't think he would need another training arc for something which is effectively the same. he didn't need an arc for the mini-rasenshuriken and other variations for example. and neither did sasuke to master enton.
now to the defence. yeah, both alone are vulnerable, although naruto's sm defence is better than jiraiya's. its hard to know exaactly though as he's been shown getting hit in that mode extremely little. sm was pieced once by nagato, km naruto was 'hurt' a few times, although he's managed to stop swords and the like and tank raikage hits without evidence of physical damage.
but the combined defense, a massively increased durability + the chakra cloak, that will take something special to get through it, especially physical attacks. do i think it can tank amaterasu or a susanoo arrow, no. but a chidori, yes. obviously some other jutsu like kamui or tobi's warp, it doesn't matter how strong your defence is. it's still vulnerable to a lot of techniques.
but as far as strength and stamina goes, yeah i think 'the sage's body' is an apt comparison, even though he may not have reached the level. madara and nagato had the sage's eyes and they were not of his level either.
anyway, my whole point is that far from being irrelevant, km+sm will end up being greater than the sum of the parts.