overall, i think you have a very good grasp on all of it.
the eq/sound processor is personal preference. i have seen setups go both ways... as a i sure you know, in theory, a eq/sound processor should never introduce any noise into the audio path... but reality is a little different once ground loops and other factors like power wire resistance come into play.
as far as i know, there is not any way to directly control the volume control within the tablet through any add-on module. there are module's available for windows pc's, but none that i am aware of for android--so far...
this is where the eq/processor comes into play-- as this becomes your primary volume and tone control(just leave the volume on the tablet at 100%).
the other option is to use something like the jl audio cl-rlc-- a volume control preamp(search youtube for soundman car audio ipad installs--i'm not affiliated with them , but they used them extensively in many of their ipad installs). as a user of 4 of them(review link), i think they work great, but at $50 ea, they are not a option for everyone.
overall, i think the eq is going to be your best option in terms of price and features. for a sound processor to equal that, you would need to step up to a model that has a external volume control-- i believe the bitone is the first option @ $900? anyways, back to the eq,i have started to develop a soft-spot for clarion units, as they always seem to have the features needed for tablet installs(usually have a basic 3-band eq, balance, fader, and volume--most other brands skip the balance/fader). while i have never used one of their eq's, i do use a clarion 18db/oct active x-over for my ipod input, and it has been very solid(if only i could get the settings right).