Most of those high mAh batteries are knockoffs from shady Asian factories and while some may be able to live up to said claims it will be a "for the first 10 charges" kind of deal (rechargeable batteries are an odd thing and will become as complex as any other technological field).
"Stacking" batteries is entirely possible (I did if for some time and redo lots of other electronics to such an end all the time)- run them in parallel and the effective charge capacity will increase approximately likewise. So will the charge time though unless you rig up something fancy and you should also remember that batteries are quite heavy (take one out of your chosen electrical device if you want to see- mobile phones/cell phones are great for this) which is not so bad but for kids and extended play it does not help and perhaps more importantly where you decide to hang the battery can change how you grip the device and the moment it exerts on your arms (think holding a small book at arms length).
Those grips are odd and frankly if you are considering stacking batteries you can probably do better- those packs will (I have seen all manner of odd designs there)-
Work-yes
Would I buy one- only to use as a shell.
Similarly the DS takes 5V in which is USB voltage and while it might not do well for signals the resistance should still be low enough to make an extended lead viable. Aside from docking stations wireless power is not really any good for anything at this point in time.
