Can ARM MBED be legally and robustly used in commercial products?
(2) All the following said, the Arduino platform would very probably do what you want easily and well. Just don't let Olin know I said that :-).

(1) But:

The MBED O/S is free for commercial use under the Apache 2 licence. 

This MBED information page addresses your main questions. It says


The mbed platform provides free software libraries, hardware designs and online tools for professional rapid prototyping of products based on ARM microcontrollers.
The platform includes a standards-based C/C++ SDK, a microcontroller HDK and supported development boards, an online compiler and online developer collaboration tools
The SDK is licensed under the permissive Apache 2.0 licence, so you can use it in both commercial and personal projects with confidence.


and


Worldwide Developer Community: Using mbed means a huge shared context with other developers, and that means when you have a question, there is less pre-amble, less explanation and less time reproducing issues, and more time getting answers. We're proud that this has helped us grow an active and friendly community of skilled developers that are collectively helping get prototypes made even faster.




For a good hands-on feel of how it really works in practice you can get an idea of developer issues and experiences, pro &amp; con, from this developer forum page



Their partner list from the main MBED home page should give you reasonable confidence in the bona fides of the system, and the rest of the claims of that page are reasonably convincing, if you feel you can trust the general level of competence of the partners involved.



I seem to have heard of a few of these :-) :

Alcatel Lucent   Atmel   CSR  Ericsson  Farnell Freescale Semiconductor IBM KDDI Marvell MegaChips Multi-Tech NXP Nordic Semiconductor, Renesas ST Salesforce See Control, Semtech Silicon Labs, Stream Telefonica   Telenor Connexion, Thundersoft u-blox WoT.io, Zebra Technologies



Home page
Developer site
Overview
Available hardware platforms - Wow!
Wikipedia
They note:


Applications may be only developed for the mbed platform using the mbed online IDE, a free online code editor and compiler. Code is written and compiled within a web browser, and compiled on the cloud using the ARMCC C/C++ compiler. The mbed IDE provides a private workspace with ability to import and share code with distributed version control, and code documentation generation.


MBED IOT
ARM IOT starter kit
IBM MBED IOT offering