While I think the order of rolling dice, keeping dice, exploding, Strife, Opportunities, Success is well laid out in the Core Rulebook, there are a few gray areas when it comes to abilities that modify dice.
The reason is that there are two places in the order at which the Kept Dice have been decided: Afetr step 5, and again after the explosive dice have exploded in Step 6.1, for I have to decide for each exploded new die if I want to keep it or drop it.

1) The Yoriki title ability allows you to change Kept Dice with a Success or Explosive Success result to Opportunity. Likewise, the Kuni Purifier changes Kept Dice with Strife Symbols to simple Success results. The question is: Does this happen after Step 5, so before any dice with Explosive Success have exploded, or after Step 6.1, so right after exploding those dice?
Personally, I prefer to let the player roll all their dice including explosions to see what they have, and then modify them. But that certainly is more powerful than having them decide what to change in Step 5, which would mean that a die cannot be first exploded and then changed.

2) Likewise, the Still the Elements kiho allows me to change an enemy die to blank. Can I change an Explosive Success die to blank before it explodes?

Another thing that came up yesterday at my group, but that is clearly laid out: In Water stance, if I have a roll with two Strife symbols and 1 Opportunity, I can spend the Opp to heal the two Strife, since Strife are converted before Opps.

So, don't know if this is an official ruling, but I would rule on my game table like this:

1. For stuff like the Yoriki or the Kuni where you change your dice faces post rolls, if they wanted to change the successes and explosive successes to opportunities after everything was rolled and kept, I would be fine, so, let's say, TN is 3, they keep 3 explosive successes with strife and roll 3 more successes, I would be okay with the yoriki changing the explosive successes to opportunities and thus keeping the roll 3 successes and 3 opportunities.
2. Well, in this case, you could change the explosive success the enemy rolled but only after he rolled the extra die and decided if they would keep or not the extra die.
3. You are correct, that's basically how it happens because strife is added before opportunities are resolved.

One could argue that since the step isn't defined, the player can use the ability whenever there is a kept die, either in step 5 or 6.1. So he could decide to use Still the Elements right after the explosive success is rolled, before it exploded, but use the Yoriki ability later after the explosions.
Same would go for the GM if the NPC have the abilities, of course.

Agreed.