So, i am left with no choice now but purchase an Expansion or a Mini Dock and obtain serial access to Omega.
This is not true at all. You don't have to rely on the dock for serial connection, you can use any 3rd party 3.3v serial solution, for example a cheap CP2102 dongle (though you'll typically get it a lot faster if you buy a $6 one than a $2 one). Just connect the appropriate transmit and receive pins and ground.
Building working firmware from source is quite complex; there are threads on it here, but after months and months of broken promises Onion is still yet to release of sources that actually work. Also those for the Onion 2 are based on LEDE, which is a derivative of openWRT, not openWRT directly.
You can however build Mediatek's version of OpenWRT for their LinkitSmart board and probably run it on the Omega 2, if you change the serial port assignments and perhaps make a few other adjustments.