Answering your question point by point, I'll try to keep it brief.
For Yvona, the fastest (and the most expensive) way would be to use the 50 seals that are used to help raise her renown. Otherwise, just hosting/participating Blessing of Cairbre works. Sadly, there isn't a "fast" way to get to the highest renown without paying, just like the other grindy aspects of this game.
Festia: remember those anniversary events where we had those mini-games? Now it's a permanent content. I haven't bothered to unlock the achievements, but from the information I can gather, you can get things like Festia Merchant Outfit (for accomplishment), the set, accomplishments with references (for example, "Rampaging Milletian" and "Unstoppable Milletian" are references to League of Legends kill streaks), etc.
Refer to this link for more info. If you want to play multiplayer mini games, stay in the most populated channel, for obvious reasons. Good idea to call your friends too.
Mirror IS the easy version. I don't know if you tried to solo it, but it's not something that you can go in and just solo ezpz right away. Basically you need MS, Fire elemental gears, and some reforges. If you're going for time, you can alternate WOTG and FH and Judgement Blade to clear real fast. Otherwise Firebolt spamming does the trick too. Now if you want to get the Phantasm Breaker (soloing the real thing) title, that's a different story.
Overall dungeon revamp is very rewarding compared to before imo. Even if you're not gonna get a good reward, you can use the dungeon to level up yourself and/or your pet since the revamped AAHM or RAHM give so much exp now. But only thing truly worth to get from the new celtic weapons would be the shurikens, as it offers significant max damage boost compared to the other shurikens. In fact, you should ditch your Hagi right now and go for Anju if you're not going for the celtic shurikens. Others are not worth it because none of the celtic weapons that are out are used for main damaging skills. Celtic knuckle = LUL, Celtic Control Bars: kinda pointless when all you do with puppet is crisis, Celtic Cylinder: if you're only going to use Raincasting or Hydra, why not just get Hurricane ego or Tower ego?
No mobile app for squires, and don't think there will be one.
Yes to both.
We did not get the KR ninja update, but nothing really terrible.
There are production success rate potions, not sure about quality. But some things do have low success rate (like the step 7 stones), prompting users to make the mc/engineering boost potions. Not sure about it being weird, but recent update includes race specific passive and active skills. Humans get the short end of this stick, while Elves get the best of this. Humans get 50 flat movement speed from the active skill, while Giants get 10% increased damage with close combat weapons (15% after the future revamp), and Elves get 400%(!!) aiming speed (basically free final shot), and this skill lasts depending on how high your total is. Many people in NA don't realize it, but in KR the Elves and Giants are ripping it apart right now. Since we're not at the time where Giants revamp is here, we're at the stage where Elves should be ripping contents apart, and if you do manage to gear up and raise one right, you can do so. Basically, it's Elf meta right now.