Those both sound like interesting alternatives. I can see some pros and cons with both.
Deft Manoeuvres appears to have a wider spread than those feats it replaces, allowing a greater range of application than any one of those feats, but it also seems underpowered compared with any one of them. While that makes sense in terms of overall balance, it may not be so good if he only wanted one of those combat manoeuvres as a preferred fighting style. I note that Improved Trip grants an extra melee attack, which I'd guess may be the principal reason for choosing it. That isn't there in the new feat, so all it really gives is the avoidance of the attack of opportunity, which can also be avoided by using a weapon or taking Improved Unarmed Strike, and the reduced +2 bonus on the check. I have to wonder whether that is enough to warrant taking it instead of something with a clear benefit.
Mighty Deeds of Arms sounds like a nice way to add some colour to combat style and descriptions. I'm guessing there's a typo in there as rolling 3 or higher on a d3 is just rolling a 3 (if you meant d4 that works much better and fits the progression at the end). That does make a difference as it either means roll max on the die or roll above half, which is the difference between useless and useful. I have two or three concerns with it as a feat. First, the progression seems costly; that's a four feat sink to get maximum effect, of a 50% chance for an extra 6-10 damage. Second, how does this feat work with all the other combat/weapon enhancers that give bonuses to hit and damage (Weapon Specialisation etc)? Four of those chosen well could easily match this on every attack with no chance of failure. Finally, as stated there is no limit to the number of times it can be used, so is the idea that it can be used on every attack roll? If so that makes it worth having, but also means it will become commonplace, rather than the Mighty Deed it purports to be.
I'm not saying they are bad ideas, but I think there are some possible flaws that need to be thought through quite carefully.
Also, I'm happy to hear that Arbuthnott is thinking of playing a fighter. That's an area where we are quite weak (no disrespect intended, Acker) and having two fighters up front will be a big help for the party. Hopefully this one will be sufficiently different from Acker that they will complement each other more than overlap.