Yes, sRGB is the best color space.
Yes, sending all those megapixels to those various devices is probably a waste. How much of a waste will depend on the software of the device. The problem with sending all those pixels is that the software will need to resize the photo(s) down to show them, which will reduce their quality due to the resizing software not doing any sharpening of the image after resizing it (which is always required for best results after resizing up or down).
Your best bet is to have LR resize the images down to what the devices will use as LR will apply the needed sharpening (as long as you turn it on).
The resolution field is meaningless; you can set it to whatever.
You will likely benefit from more than one preset.
1. Yes to sRGB
2. You haven't stated how many megapixels you are using. 16:9 is a ratio. If you crop just the top or bottom of an image to fit the ratio it will look fine on an iPhone 5/6 while there may be black bars on other devices.
The more important factor is the actual number of pixels you need on the longest edge. As an example the iPhone 5 has 1136pixels by 640pixels. Thats a 16:9 RATIO. Having more pixels at the same ratio serves no benefit.
Depending on the devices you want to see the images on i would pick either the largest dimension and resize the images to that. If there is a significant difference to the next device (eg. 4k tv to iPad) I would create two separate export presets and name the files accordingly.
As ManiacJoe suggests. Resolution has no bearing in this case.
When I am preparing images for display on my television I first crop them to 16:9 aspect ratio. Then I export them with the long edge set to 1920 pixels. Resolution setting has no bearing. The images always have displayed well.
I was always under the impression that monitors and display devices only have resolutions of 72 so anything above that is not displayed and unnecessary . At least this is what it use to be with web graphics. Is this not the case any more?
It hasnt been the case for a long time.
A pixel is a pixel. 2000 pixels will take 2000 pixels to display. the issue is that of the device. The native resolution of a 20in Apple Cinema Display is 1680x1040 so a 2000 pixel wide image will be too large for the display at 100%
By contrast the iMac 5K has 5120 x 2880 so the same 2000 pixel image would take less than half the screen.
The "per inch" resolution of 72 240 or 300 only relates to the inches on a sheet of paper, not a screen.
You can set the resolution to any number that you want. If 72 makes you feel comfortable, then do it. But it really doesn't make any difference. I have found that an image of 1920 x 1080 pixels fills the TV screen perfectly. And it doesn't matter what the resolution setting is. The image has the same number of pixels regardless of what the resolution setting is.