Everything you described is NOT default behavior. The default behavior is for iMessage not to be on. If you have an iPhone and don't turn on iMessage then texting works just as normal.
To be clear, you have to actively assign your phone number to iMessage, and then assign email addresses and devices to that account.
The whole point of iMessage is to disassociate your phone service as the controller of your SMS and have the control be given to iMessage. This is in part so that you CAN send and receive texts on things like your MacBook over WiFi with no cell connectivity and all transparently.
Sure, I can see how it may be confusing for people who stick their heads in the sand and wave their hands in the air when it comes to reading instructions, but changing the way the system works to accommodate the ignorant isn't the answer.
That's like blaming Google hosted email for hijacking a domain's email.
Why did your wife's iPad get the text message? Because she configured it to do just that. Same goes for your coworker's MacBook Air. As far as your niece goes, the phrase "after she wiped it" is false.
This comes from someone traveling in another country right now who just had to send several iMessages from my MacBook Air that would've cost $$$ in international texting had Apple not set things up the way they did.