At the technical level, where ACR
fits in is that you need it if you want to open a
raw(ie. in your situation, the NEF files) in Photoshop.
Lightroom can view and open NEF files without ACR, and I think this is because it's basically a hot rodded version of ACR anyhow.
Photoshop itself can't see
raw files, so they have to be translated into an acceptable format before you can open them in photoshop.
So in a sense, ACR is similar to LR in the sense that: if you want to use photoshop, you first need to open the NEF file in ACR/LR it will convert it(I think into a psd file, or something like that) and then you get it in Ps to work on.
You can obviously save the edited image in various formats, and I think Ps's strength is in saving it as a psd file, but you can't really save it as a
raw file again.
I haven't used Bridge all that much, but I don't think it can actually do the conversion from camera manufacturer
raw formats into a format for use in Ps. If you open with photoshop via bridge, it sends it to ACR, then you send it to Ps.
I remember having a lot of trouble with ACR via Bridge .. dunno what happened, but I got error messgages saying ACR wasn't installed(???). I opened ACR directly and then the NEF file and all was OK.
But this did only happen after I updated ACR(only) to a later version of some type, but not Ps or Bridge.
So in a sense, there is a kind of heirarchy, but it's not really a heirarchy as such.
So if you choose to use LR, you don't really need anything else. It's a browser(ie. like Bridge), it's a converter(ie. like ACR), and it's an editor(ie. like Ps).
**Note that while ACR can edit, it's not really 'editing' in the sense that you can do localised adjustments. So in this situation it's really a 'converter' .. whereas Ps and LR are 'editors'(ie. the ability to do localised adjustments as well).
I didn't find Adobe products 'intimidating' so much as they were(are!) frustrating.
Ps is just a case of seriously sticky BS!!
.. So many idiotic(ie, frustrating) edit steps made non intuitive by standard procedures used by most(if not all) other editing programs.
And LR is just plain annoying if you just want to open and edit a file you want to get onto your computer ... with it's stupid import/catalog silliness!