You should always use the latest firmware, but stick to UC 1.8.1 if you want to use FireWire. If you don't need FireWire, go for UC 2.4.

As the 16.4.2AI has no dedicated On/Off button for the equalizer, as 32.4.2AI and 24.4.2AI have, this function should be by-passed in UC Surface, or always set to "on". You can check this very easy: Load the "Zero Out (Board Reset)" scene, now the equalizer should be "powered on". If not, this is a bug...!

It should be normal to use a computer or iPad with a SL AI mixer. In the case of a SL 16.4.2AI UC Surface gives you access to the full four band parametric equalizer (including "Q"). But you need to be aware of conflicts between UC Surface and the mixer. Use EITHER the full parametric EQ of UC Surface OR the semi-parametric EQ of your mixer. You can also use UC Surface for "fine tuning" of settings you made via the encoders of the mixer, but then do not touch the encoders again.

You know that all settings are saved with scenes and that you can choose which settings you want to recall when loading a scene. If it happens that the EQ gets "powered off" and this is saved in a scene, the EQ will be off and you NEED access to UC Surface to turn the EQ on again. It is recommended to recall the factory or your own "Zero Out" scene often. This helps preventing malfunctions.