Shinto is Japanese martial arts, and a bit more. Primarily I see J martial arts as the two handed sword, which is the most powerful J weapon prized by Samurai`s.
Again, only latterly, I would say. The great sword schools did not emerge until the late fifteenth century. Originally the Way of the Warrior was 'The Way of Horse and Bow', the sword was not synonymous with the samurai then, that happened much later. In battle, the weapon
par excellence was the spear, until the arquebus did for blade weapons.
I was a sometime-exponent of Muso Shinden Ryu swordsmanship, so for me the sword is the business! At a demonstration of martial arts, the sword always takes pride of place. But give the bow its due ... I once watched a demonstration of yabusame (horseback archery) ... impressive!
The Japanese sword is made accordingly with Shinto traditions, no buddhism there. All the other J martial arts are probably of buddhist origin originating from Karate (boxing, ninja etc..).
The sword, yes ... I'm not too sure about the other stuff. But then the history is very clouded.
The great Ryu taught 'flexible skills' (wrestling), and other hand-to-hand combat skills, of which Sumo is a derivation. If you're having a punch-up with a bloke in full armour, the best (and maybe only) way is to get him on the ground, then stab him (think of the Sumo grip).
Again, ninja training is steeped in shugendo lore, and the practitioner of shugendo (shugenja) were also known as yamabushi (one who lies in the mountains), from the inhospitable mountain regions where the sect of austere asceticism flourished. Yamabushi were almost legendary in their martial powers, a mix of Shinto, Tendai Buddhism, mikkyo ...
Senei Usui, 'founder' of Reiki, was a high-ranking master swordsman — he served in a lowly position in the Japanese delegation to China, which makes him a bodyguard or a spy or both in my book — and his realisation came after practising a 20-day Tendai ritual.
(In recent history, Paddy Ashdown, a British politician, was a one-time member of the Special Boat Service (SAS in boats), a linguist who knew numerous Chinese dialects. He turns up as the second-reserve-window cleaner or something at the British Embassy in China ... a spook, for sure!)
There is no doubt that Buddhism played into the martial training methodology — the samurai would use anything that works! Same with 'imported' empty hand skills from China, Okinawa and elsewhere. They learnt the art of metalurgy from Korea, Japanese swords had a tendency to snap until they followed Korean methods.
As you point out, the Tokugawa Shogunate utilised Confucianism to 'restructure' the samurai ideal, along with Buddhism and Shinto. Zen [layed a big part in this, and was already influencing the sword schools.
BTW:
Are you sure about Christians implicated in Oda's assassination? He was their best bet, surely? Without him, they had no real support and a lot of opposition. After his death, their position deteriorated rapidly ... but sheesh, our missionaries were also political meddlers, for sure!
Oda Nobunaga was a mercurial and probably unbalanced personality, you never knew where you stood, and he treated his vassals very badly, by all accounts. Akechi Mitsuhide suffered more than most, and it must have come as no real surprise when Akechi attacked Oda.
I know history is written by the victor, but by all accounts Tokugawa Ieyasu dealt very fairly with his retainers, never betrayed any of them, and stayed true to his word. As a result, from the age of 16 on he began to attract vassals who remained loyal to him throughout, so that when he fought his later major battles, notably the 'big one' at Sekigahara, he had the advantage of a unified force opposed by an uneasy coalition (of which a significant proportion deserted to the Tokugawa as soon as they got a sniff of his likely victory).
Anyway, if you're still awake, thanks for jogging the old grey cells. I haven't thought about this stuff for years.
Thomas