HDMI is a complicated beast and while there is a standard very few manufacturers submit their products for testing. To understand ARC you have to understand how HDMI communicates. Before I go any further both ARC and optical connections are capable of 5.1 signals. Whether they use them is a different story.
In the HDMI chain the source is the master and controls what is sent. Everything has to authenticate itself to the source. So your cable/satellite box, Blu-ray player, apple TV, game machine etc. are the master in your system. The display has to authenticate with the master and here is where the problem with ARC and optical connections from the TV might only send 2 channels of information.
The TV authenticates with the source and states it is a stereo device. The source also know there is no receiver in the chain so it sends a stereo signal to the TV. The TV connected to a receiver or sound bar via either ARC or an optical connection passes along the stereo signal. So you do not receive 5.1 information. So for the TV to pass a 5.1 signal it has to identify itself as being capable of receiving a 5.1 signal. For TVs to receive a 5.1 signal they must have a 5.1 decoder in them.
That is why ARC or optical might only get 2 channel PCM because the TV is identifying itself as a stereo only device. Depending on your equipment you may or may not get a 5.1 signal. A better solution if your sound bar or receiver is capable is to connect all your sources to them and then run one HDMI cable to the TV. If the audio device is in the middle of the chain it will be identified by the source as being capable of multichannel sound. Also, you can receive lossless audio. Something that is not possible through either ARC or an optical connection with 5.1 or 7.1 sound.
Beside ARC not being able to receive lossless audio another disadvantage is to use ARC you have to turn on the CEC function of HDMI. CEC stands for consumer electronic control but each manufacturer uses its own name for it. Here are some.
Panasonic - Viera Link
LG - simple link
Sony - Bravia sync
Samsung - Anynet
Anyway, what CEC is supposed to do is turn on the devices needed, switch everything to the correct input and have the volume control work for the system. So for example you are watching TV and then turn on your Blu-ray player. The TV and your audio device switch to the correct input (and turn on if off), your cable box shuts off and the volume button on your Blu-ray play now controls your audio system. Well, this is an area where manufacturers use different codes for different functions and these things do not play nice together. Even devices of the same brand I often can't get to work with CEC. With VERY few exceptions it is probably just better to not use it.
If I were to give some general advice. Do not use CEC and ARC, turn them off in all your devices. Connect your sources to the audio device if it can do HDMI switching and then connect and HDMI from the audio device to the TV. If not run separate audio cables to the audio device and HDMI to the TV. Get a decent universal remote to control the system, they are not that expensive. If you follow this simple advice you will have lossless sound, good control of the your system and will decrease yours and your family's frustration.