It definitely does not form a "perfectly comfortable straight line".
And are you using Flat / Convex buttons... or are they Concave?
Its FAR more comfortable pressing a Concave button with a Vertical downstroke, rather than with a flat hand !!!
That COMPLETELY eliminates the very REASON why there is a 'finger cradling depression'. In using a button this way.. your going to be constantly pressing your fingers into a hard pointed edge. And how exactly is that more comfortable?!
Not only that... but the very use of flat or convex buttons is non ergonomic, and far more fatiguing on the fingers.
1) With the hand flat, the fingers not having a bend at the knuckle to provide the spring-force, you lose:
a) mechanical advantage
b) You lose press power
c) You lose reaction time
d) You lose dexterity and fast accurate presses (try drumroll type exercises spanning 3 keys)
e) You will fatigue much quicker, because of these losses
2) Please show me, and or design a keyboard that fits everyones flat laid out hand. (good luck!) And then, have them type on it to get the most words per minute.. and do fatigue tests. Ohh wait.. none exist.. and thats for good reason... reasons listed above, and more.
(hint: you cant design such a thing, cause everyones hand shape & finger lengths are different... thus, one more reason why flat handed buttons setups are POOR for human ergonomics, and thus especially bad for accurate gameplay! HOWEVER... a vertically pressed button setup... can be adapted to ANY size hand or finger length)
asking a serious SF2 player to play against you with your custom, curved buttons is borderline rude. Might as well hand a rapier to a samurai, then ask him to duel you.
I completely agree!

as you spread your fingers apart they revert back into a curved arc.
That depends on IF you chose to let the middle finger drop down, or keep it vertically bent at the knuckle. I feel most people would prefer to keep the power of that digit for downward force, response speed, less fatigue and accuracy.
By placing your fingers on S, D & F.. keep the middle finger on D, and just move the two outside fingers one key over to a & g. Note, that you dont have to lose the Bend in your middle finger.. and even the side fingers still remain about the same height. A mm at most in my test. The only real change, is that they roll over slightly on an axis. Therefore, the downforce and leverage, is pretty much the same.
Try placing 4 fingers on letters H,J,K,L... Easy right? Now place the same four fingers on D,G,J,L... Not so easy eh? Now try D,T,U,L... Which is more comfortable?
WHY would you place 4 buttons in a straight line, and expect them to be all hit at the same time?
The 6 button layout, is for SF style fighting games. Spanning 3 arcade buttons is easy for most, especially in that you dont do it repeatedly like a machinegun. If this were that difficult to do.. then nobody could play a simple chord on a keyboard.
As for 4 buttons at once... most of those games are laid out in a Tekken 2x2 array. And games like Samuri Showdown, could very well be played the same way. However, in the event that you want to play SS with a straight layout.. add that 7th button at a slight drop, as seen pretty much all over the place here... which allows the pinky to use the last button effectively. (being that, unlike the cm size difference of the middle and side fingers... the pinky is about 2x that, a full inch in height difference)