As long as I can understand, you will have a custom Android ARM board that will communicate to your embedded controller via RS485 or SPI. I said "custom" because AFAIK the standard Android platform/device usually just supports USB as a physical link. In the future, you plan to use a standard Android tablet/smartphone as a "console" so you will have USB communication only (or you will fall back to wifi or Bluetooth).
In this case, I think we can assume that you will end up using the Android Open Accessory Dev Kit at the end of your development process and will use USB as you physical link:
https://en.wikipedia.org/wiki/Android_Open_Accessory_Development_Kit#Android_Open_Accessory_Development_Kit
http://developer.android.com/tools/adk/index.html
This will be a source of headaches by itself.
If you plan to develop for a few different platforms (mainly Android, Apple iPhone and Window 8 Phone) most likley you will end up using an abstraction layer like:
https://en.wikipedia.org/wiki/Phonegap
http://phonegap.com/
https://en.wikipedia.org/wiki/Appcelerator_Titanium
http://www.appcelerator.com/
or any other one of these:
https://en.wikipedia.org/wiki/Multiple_phone_web_based_application_framework
This will be another major headache.
Given this scenario, learning HTML5 technologies (that actually means HTML5, CSS3 and Javascript) will be the easy part.
Maybe you could evaluate a differente solution. Maybe something similar to AirDroid:
http://www.airdroid.com/
That is: put a small web server inside your embedded controller and use it to expose a web interface via wifi to any web-capable device out there. If you are using Linux, QNX or any other Unix flavor as your embedded system OS, the embedded web server should not be a problem.
This way you would have to develop just a single administrative web application to be hosted by your controller and an single web-interface for your clients. Please note that this client-side UI would not be a HTML5 wrap around the native mobile platform (like phonegap) but just a simple web page that could be seen and used by any web-browser on any kind of device.
Have a look at webmin for Linux, as well:
http://www.webmin.com/
https://en.wikipedia.org/wiki/Webmin
There are other, sometime better clones of webmin out there. Many of them are already used in DSL routers and other net appliances to expose a web interface via ethernet and/or via wifi.
To expose your embedded controller functionalities to the web-admin application you will just have to develop a few interface libraries (C/C++ to Perl/Python/Ruby/whatever). This can be done with tools like SIP and SWIG:
http://wiki.python.org/moin/SIP
http://en.wikipedia.org/wiki/SWIG
http://swig.org/
Put aside these technical considerations, learning and managing HTML5 (actually a mix of HTML5, CSS and Javascript) can be done in a very short time. Most likely, you will be able to "parallelize" the learning process.
If you use a layer abstraction, like phonegap, put at least a couple of your most experienced programmers on it. This will be the hard part (because phonegap is the crossroad of all of the other problems). Ask them to develop a small, very simple app with it. It will take them a week or two but it should be enough to understand how to use the abstraction layer and how to solve the main problems.
Javascript is a strange language, with very good parts and very bad ones. Most likely, you will have to put at least one or two of your OOP programmers on it. Ask them to develop a small demo, like a small calendar application. This should be enough to see the most important issues of the language and should take them just a few days or a couple of weeks.
Put one or two of your structured (C) programmers at work on HTML and CSS. This should be quite easy for them. Please note that they will not have to dominate CSS. They will just have to understand and use it in a quite basic way (at least at the beginning).
Most likely, the Javascript and HTML/CSS teams will have to work together (because JS cannot be used without the HTML/CSS stuff).
Have each one of your small team reporting what they have learned to the other teams once a week. This is required to merge together all of the required skills (at least at the conceptual level). Have them develop a very small fake protototype of your app before beginning the real project. You have to develop a HTML+CSS+Javascript+otherstuff application and it would be very hard to develop the required skills without a small test project.
In any case, your real problem seems to be the lack of a senior programmer who already knows all of these technologies and can lead you in the design stage (and can solve the hardest problems during the development).
ORION is absolutely right in suggesting you to hire a consultant to play this role.