1. Why dumbfuck Ace besides the flashback never used haki and the WBs never teach them to him probably…
2. How strong he would be if he would have the training Luffy had.
3. Why Rayleigh never tried to meet him or even save him. Seriously... He was swimming through the Calm Belt to see Luffy... but he didn't go to the war?
1. Maybe he did know haki. He was aware of what it was when Luffy used it and I'm sure he was curious about how people like jinbei and WB could hurt him. Speaking of which, Jinbei had haki and it didn't help him much against akainu either.
2. Just because we see him lose against BB and akainu doesnt mean he's not strong. I'm willing to bet that Luffy is around Ace's level now.
3. Rayleigh barely knew Ace. Besides, it wasn't his fight, it was Whitebeards. Like Ace said, pirates have their own paths. Also he is basically an old hermit.