What you're describing is called Active Lane Control in Infiniti-speak. It is different from Lane Departure Prevention that was available in some Infinitis. LDP (which has LDWarning as a pre-requisite) was not an option on 2017 and previous QX50's.
LDP uses braking pressure to attempt to keep you in your lane - it has very limited capability and at best, it will have you ping-ponging down the road. IMO, LDP is almost pointless. Well, not pointless, but not a feature I find very useful.
Infiniti first introduced Active Lane Control in the 2014 Q50. It requires Direct Adaptive Steering in order to function. DAS is the steer-by-wire system that by itself is just a cool tech thing. Its the ALC that it enables that makes it truly cool. DAS also makes steering very adjustable and feels great, IMO. The 2014 Q50 was DAS v.1.0. The 2016 Q50 was DAS v.2.0. I'd call the DAS in the new 2019 QX50 v.2.5 (Infiniti probably doesn't use this naming scheme - its my personal shorthand).
I've had Active Lane Control in my car since my 2014 Q50 Hybrid. I now have a 2016 Q50 Red Sport with it and I love it. There are a couple of subtle, but important differences between the implementation of DAS/ALC in Q50 vs. QX50.
The biggest difference in terms of using the feature is that the new QX50 requires you to keep a hand on the steering wheel. In my Q50 I can go fairly long distances (assuming a pretty straight road) without touching steering brakes or gas. In an experiment, I once made it 12 minutes (15 miles) like this. In practice however, I almost always keep a hand on the wheel. In the QX50, if you don't touch the wheel for 20 seconds, the car will start warning you to put your hands back. If you ignore the warning, the alarms will get more insistent, ultimately stopping the car.
Also, in the Q50, steering will operate independently of cruise control. In QX50, they're tied together. You engage both simultaneously. On the plus side, steering and cruise control work at any speed in the QX50. In the Q50, steering works only above 45mph.
Either way, you should think of ICC and ALC as highway functions much more than around town functions.
The ALC in the QX50 is actually more capable of dealing with curves than the version in the Q50. Its also able to continue doing its thing all the way down to a stop, along with ICC. Also, ALC in the QX50 will move the steering wheel - it doesn't in my Q50.
In order to get this capability in the QX50, you need to get an Essential model with the ProActive package. ProAssist gets you ICC, only ProActive gets you ALC. If you're looking at AWD, you should find they mostly have a MSRP of ~$51-52K (subtract $2K for FWD).
My lease on my Q50 ends late next year. So I don't have to make a decision yet on features. But if I were going to get a new QX50 right now, I'd undoubtedly get one with ProActive and Premium Heat (and probably Audio). My only debate would be whether I'm willing to spend the extra money for the Sensory Package. Its a tough call because sticker price jumps to $57K. For that, you get nicer leather, suede accents and headliner, real wood, kick-to-open tailgate, great looking wheels, passenger lumbar, rear sunshades, headlights that turn when you turn the wheel, and a better climate control system. Kick to open tailgate is a gimmick that looks awesome in TV ads but doesn't change much IRL. Cooled seats are wonderful. Wiggly headlights are cool and will impress your friends. But IMO, they need to add one or two cool, functional things to the Sensory package for me to spring for it. It adds ~$65-75/mo to a lease payment.
Did I answer all your questions?
PS, if you're in NorCal and your salesperson can't explain all this to you, I'd love to earn your business. My contact info is below.
