You got already good advice for your own questions.
In regards to your Girlfriends goals:
"Finally, my girlfriend is currently losing body fat and is nearly at her ideal weight and body fat %. But she wants to get a flat stomach with firmer butt and legs. Obviouslt she needs to be doing deadlift varieties, lunges and squats but should she be eating at her BMR, above or below it?"
Itis actually not so "obvious" that she should be doing deadlifts, squats and lunges to reachg that goal.
Deadlifts only help for glute developement when done in the correct form. You'd be surprised how many people do deadlifts with completely 'muted' glutes (means the glutes not being evolved enough) and just pulling it through the lower back. But if she does them right and sets the body up in the right positins throughout the full lift, then they 'can' be a great glute developement 'helper'.
Squats are a good coice. But also here form is the key - that makes the difference to which muscles have the most benefit from it.
For ladies additionally weighted donkeykicks on a cablemachine can help greatly to develope better glutes.
Same for the vertical legpress. (but only if full vertical, the incline ones mostly just go onto the knees, quads and hams)
Hip extensions against a strong rubberband is also a good choice.
As well as barbell bridges, which is probably the strongest glute developer of all the above evercises.
Legs will improve with all the above as well.
Flat stomach, here is the deal. Most people also lack involvement of their abs in their exercvises. If you know how to embrace your abs to get on your 'inner belt' while executing exercises, you already accomplished 15% of the way to 'flat stomach city'. Another 15 % are exercises for the abs. This can be weighted situps, hanging leg raises, cable pulldowns and such, even OH presses when heavy enough to have the core balance the weights. (but crunches alone are for sure not the way to a flat stomach ^^) .. but now comes the most important part... The other 70% to wether your stomach ends up being flat or not... are done completely alone in the kitchen.
Food choices and how much you eat are the main factor to flat or not flat stomach area.
On top of that comers genetics, especially for females, the storage around the stomach and the glutes / thighs, is a typical thing. And often means you have to get to ridicilous low bodyfat levels to accomplish this, especially when you want the abs to show. So that's a personal choice, and a choice of eating :)
Clearly if she doesn't want to 'put on weight' she'll have to stay at her maintenance level. Possibly even a bit below if she still needs to shed fat on the stomach area.
Hope this helped :)