It gives you an extra opportunity to spend a ki point for one extra martial arts die's worth of damage (potentially ranged) per turn
You seem to have two major concerns:
1d6 is rather little damage compared to the other available ways to spend a ki point
The damage potential of Flurry of blows is no doubt a lot higher, and the way it is written it clearly applies to only a single attack. However, Deft strikes' trigger is when you hit with a kensei weapon, which means there are some trade-offs that can potentially make Deft strikes situationally good:
- The extra damage is guaranteed
- This can be used after you land a critical hit, which would double the dice
- You can deal the damage at range, if the weapon you're using is ranged
- Extra damage is extra damage. If your target needs to go down now, this might make a difference
When the Martial Arts 1d6 die becomes up to a 1d10 later on, it obviously does so too for the extra attacks from Flurry of blows, so that's a moot point.
You can use both Flurry of blows and Deft strikes in the same turn
Flurry of blows allows you to spend one ki point to make two unarmed strikes as a bonus action. The trigger for it is: "after you take the Attack action on your turn". Nothing bars you from still doing so after using Deft strikes.
You most likely will not be able to use Martial Arts as an animal, but not for the reason you brought up.
There are two factors to consider: the limitations of the Martial Arts feature, and those of the Wild Shape feature.
Part of the description of the monk's Martial Arts feature reads (PHB, p. 78):
You gain the following benefits while you are unarmed or wielding only
monk weapons and you aren't wearing armor or wielding a shield:
Armor and shield are obvious, most animals will not use them and natural armor is not "worn". So can you not wield your natural weapons? In an unofficial tweet in December 2016, rules designer Jeremy Crawford said that:
No general rule gives "hold" or "wield" meaning beyond the English
(the words and their meaning in a particular context).
The OED's definition of "wield" states:
Hold and use (a weapon or tool)
So if you simply do not use something, you are no longer wielding it. You can certainly do that.
The description of the druid's Wild Shape feature says, in part (PHB, p. 67; emphasis mine):
You retain the benefit of any features from your class, race, or other
source and can use them if the new form is physically capable of doing
so.
Can a bear perform martial arts maneuvers you have learned as a human? Can a crocodile? This will be up to a DM ruling, but the answer will most likely be no. The only case where I personally would rule yes is if you take the shape of an ape or monkey.
Best Answer
Yes, these will work together
As you have pointed out, your martial arts die at monk level 17 is d10, and if you pick longsword as one of your kensei weapons, then you can deal a d10 with that weapon. From XGtE, p. 34:
And from Martial Arts class feature, PHB, p. 78:
If you are wielding the longsword in one hand, regardless of how the damage die has been modified by the martial arts die, you can add the +2 to the damage as per the Dueling fighting style (PHB, p. 72):
Your longsword (wielded in one hand) will deal d10 + DEX + 2 damage.
However, also note that even without the Way of the Kensei archetype, you would be able to deal the same amount of damage if you were using a weapon that was already a monk weapon, such as a shortsword, dagger, quarterstaff, etc, so long as you wield it in one hand (for the +2 from Dueling).