Thanks for the comments, DW. 

Cost of DX: My DX (athletic only) does include DX's normal
Basic Speed adjustment – certainly, full-body agility should add to
that. So it starts at the full 20 points. (I take it my text needs to
clarify that BS is included?)  

From there, High Manual DX is 5 pts/lvl in the positive direction
(i.e., "added back" when you buy -1 DX (athletic)), but has uncertain
cost in the other direction (only Ham-Fisted to go on). No neat way to
set a cost going both directions.  And there's no known cost for the
missile weapon adjustment. I feel that a final 15 pts/lvl for DX
(athletic) is well within the fudge boundary, and has a neat cost. 

Re Move for large creatures: I want to avoid Enhanced Move as the
default method. There should be no "break" in how large vs small
creatures work, IMO; after all, from a Halfling point of view, a human
is nothing more than a large creature with bought-up Move, yet we don't
operate under Enhanced Move restrictions. The same base rule of
movement should apply to everyone as the default. 

But, I don't mind offering Enhanced Move as an option, as some big
PCs may need it purely for cost purposes. If I can make it fit, I could
add the +50% "all modes" option too.

Re special skill mods for Acrobatics, Jumping, Climbing, Swimming:
While those are part of the old GULLIVER, I think I'll leave them out
here, especially for Mini.

First, the simple fact of a DX mod does place some mod on those
skills. But why not even more extreme mods? Well, 4e doesn't place any
special penalty on Acrobatics and Jumping even for encumbrance, whereas
my DX mods stem from a lesser problem (just high mass, not high weight
& encumbrance). 4e does place an enc penalty on Climbing, and a
double one on Swimming, but again, that's for unmanageable weight, not
mass; the difference is especially clear in Swimming, where dense
carried weight should certainly cause trouble but normal-density mass
should not (i.e., whales should not have a special Swimming penalty). 

For Mini, I need to keep default complications to a minimum, and
would prefer to stick with a simple DX mod as the base that anyone can
use with ease.

HOWEVER, my DX (athletic) option branches into more varied
complexity; perhaps that could be a place to fold in additional mods?
For example: double the athletic DX mod for tasks requiring exceptional
full-body agility, like more challenging uses of acrobatics and
climbing that rely heavily on arm strength. Of course, that'd boost the
value of DX (athletic); something else should be dropped to compensate.
Come to think of it, DX (athletic) shouldn't affect vehicle skills,
should it. That may provide enough balancing factor, to keep the cost
unchanged.

Just an idea; could be added to the text if worthy, and if it fits!