OK some of my own experiences at legendary.

Firstly the Saito while supposedly hostile can be easily turned into allies and even vassals on turn 1 (both alliances and vassaldom seem to be a bad idea as the Hattori seem programmed to expand into Mino and will always invade it with a huge army (for the early game) so there is no point being dragged into that war too early) - this does give you a few free turns to deal with the Tokugawa and Imagawa.

Secondly it cannot be emphasised how vulnerable Owari castle is and a couple of Ashigaru are not an adequate garrison - so you have to keep your main army very close to it which is a problem when chasing rebels and invaders.

In my first abortive legendary campaign I moved too far away chasing the rebels and it got took by the Tokugawa in turn 2

In my second I beat the rebels and the Tokugawa but again ended too far away to defend it and an Imagawa army snuck in and took it.

Several failed campaigns later the Kitabatake stabbed me in the back and took it the turn after I'd taken Mikawa - and even though I had left my Daimyo and half a dozen part-strength Ashigaru units to hold it they sent a three quarter full stack with a lot of katana samurai who just slaughtered everything in their path - and then they followed me to Mikawa castle and did the same thing there.

So there is a strong argument for only leaving Owari castle behind and going conquering when you can leave a full strength half stack of Ashigaru to hold it.

Regarding Mikawa it is easy enough to take as the Tokugawa tend to leave it ungarrisoned but then very difficult to hold as it will have very negative loyalty and the rebels tend to be a mix of yari samurai and light cavalry which can wreak havoc on your bow and yari ashigaru unless you are very careful.

I actually had no problem beating the rebels but they did infllict a lot of casualties which meant I was easy meat for the Imagawa and Kitabatake who attacked later.

I am wondering now if turtling might work - as long as you stay in Owari castle you can beat any invader for at least the first few turns until they start sending katana or naginata samurai at you (at legendary I've never survived long enough to see any Ikko-Ikki warrior monks) - its only when you send an army away to expand your territory that you become very vulnerable indeed.

Re the tech tree I'm also sticking to the far right hand side and taking all the building and economics ones first - this does seem to have more of an effect than the military ones in the early game as you really won't be able to afford the specialist samurai and cavalry units it opens up for a great many turns anyway.

In fact these turning up very early in enemy stacks long before they should be able to build the required tech is the one thing that does bug me about legendary - I don't mind them having more troops and their being buffed up relative to mine but looking at an enemy stack and seeing units in it that I have no obvious answer to is a real issue for me (rather like the MTW2 English rebels that had longbows and armoured swordsmen long before you could recruit them).

As for trade the AI seems quite unable to use the trade nodes properly (and if it does occupy one if you wait for a few turns they'll either leave it for no reason or be eliminated as a clan and free it up.

So building a trading port and spamming trade ships as soon as possible seems a good idea.

It it is worth building several cheap warships and sending them out first - they can sit on a trade node and deny it to others and as they cruise along you can discover all of the western clans and establish early trade agreements with them all.

So although I marvel that anyone has finished it at least for the Oda (and I'd be willing to bet for the Takeda as well) CA has delivered a campaign that is a real challenge even to the most jaded veterans of TW.