Sounds contrary based on my experience. If I set ISO to 400 and Shutter speed to 1/100, and WB to "Sunlight", they stay that way until I change them. I remember because I've had shots that were too bright or too dark when moving from a dark place to a brighter place and vice versa. WB though will not be as granular as ProShot (which I also have), but even the temperature values in ProShot use the same SDK. Head over to the ProShot thread where the developer is quite active and responsive. My last communication with him went along those lines, that it is still limited by what the SDK allows to be done. Hence, if the SDK doesn't allow finer temperature white balance settings, it cannot be done.
But to qualify, even the settings in "many recent droids" are not "proper". If I'd like a proper photo with proper settings which I can properly customize to my heart's delight, I'd use my X-T1 - not my old Lumia 1020 nor my iPhone 6s. Phone cameras, no matter how expensive and "modern", simply will not match the performance of a full camera (except in sunlight where all of them are "great" as advertised).
Also, since the question was also about exposure lock, fix the eV - but that will work only within a range of ISO and shutter values (to avoid sacrificing image quality). If the eV you need isn't covered by adjusting the eV, manually adjust the shutter speed and ISO.
Well, I guess you haven't paid much attention to what you've read, but anyway, let me elaborate this a bit more:
There's no complete exposure settings FOR VIDEO, pal. Video mode only gets exposure compensation values. It doesn't help at all.
If you're talking about the Pro Shot app, it indeed has exposure settings for video, but the GAPS BETWEEN each value are huge for quite a lot of situations.
If you go along with your investigation on the Pro Shot app, you'll see that sometimes the auto mode shows you the ISO value. And sometimes a ISO like 323 is better than the 200 and 400 steps from the manual exposure menu. Get it? The same goes for shutter speed.
THAT'S WHEN THE EXPOSURE LOCK HELPS.
Yes, RECENT DROIDS do have PROPER EXPOSURE LOCK. I am sorry that you didn't get that.
Regarding your wish to use a 'real' camera when you want something better, that's your right. But I know I can get a fantastic quality out of my Lumia 950, and no matter what your personal taste sounds like (about which I could not care less, by the way), I still would like to have at least exposure lock FOR VIDEO on the default camera app. Sometimes you simply cannot carry a 'much better performer' with you. Sometimes it's awkward, you know. So it's very handy to have a capable cameraphone inside your pocket.
Besides, I have managed to get much better quality out of my Lumia 950 than many dedicated cameras, even in low light. I guess your vision of things may be quite outdated, pal. Here's one example of that:
By the way, your XT-1 is quite an appalling camera for video, I'm not sure you know it. Extremely high on contrast, limited to 1080p and with lots of moire and aliasing artifacts. And Its low light video performance is just a mess. You'd be better off getting a Sony RX10 MKII, like mine. And for the comparison's sake, the 950 would trump it in many occasions when it comes to video mode. Even when we consider the superior audio quality captured by the Lumia.
Here's a quick XT-1 (your 'proper' camera) daylight video clip (hard not to laugh at this awful performance):
EV is an adjustment for COMPENSATION. It won't lock the exposure.
Sunny white balance also won't lock the WB. It will only limit the WB to the the 'warm' white balance range, but also won't lock your WB. The Pro Shot app does lock it by changing the color temperature values.