Excellent that you worked with Sketch!
1.0) No, you do not have to design each device separately. You can certainly make models of higher fidelity this way, but believe me, it will be a pain make changes. And there will probably be a lot of changes.
There are a few different schools of thought here, but the consensus is almost on the fact that you must first design in the lowest resolution (in your case: 320×568, for iPhone 5 @ 1x). If you started with 750×1334 (which corresponds to iPhone 6 @ 2x), for example, and created a 55×55 dimension element, when you try to downsample to @ 1x, you end up with decimal points (27.5×27, 5). should be rounded to 28×28. Depending on the illustrations, it could ruin your image.
1.1) The only exception is when working with bitmap images. As you probably know, it is much more problematic to stretch a bitmap image than to reduce it. So you have to work at 4x (100%) and downsample at 3x (75%), 2x (50%), 1x (25%).
Working with 4x ensures that your bitmap images will be ready for Android xxxhdpi (which is 4x) and also minimizes the rounding of pixels, since 3x -> 2x would require 66.666666666666% of the size of the image.
2.0) I recommend designing for the smallest device that will be used. In your case, iPhone5 (320×568). Perhaps you want one of the screens to fit comfortably in the device without scrolling. If you are designing from the iPhone 6, on the iPhone 5, some content may be off screen.
Another thing is that even if the iPhone 5 and 6 render @ 2x, and both have the same size for the tab bar and the navigation bar they will have different widths. So, a tab that looks comfortable on the 6 could be cluttered on the 5. And a title that stands on the 6 could be truncated on the 5.
2.1) About the points: First, all the point recommendations are for @1 time. So a button on the iPhone 5 or 6 will have a height of 88px. Points is simply a convenient way to communicate with your developers. Instead of saying that something is 100 pixels wide, say it, it's 100 points wide, and they know that 200 pixels are supposed to be on @ 2x, and so on.
2.2) Visually, segmented controls have only 29 elevation points. I'm not sure of the technical functionality, but note that on Apple applications, segmented controls (29 points high) are placed on areas of 44 points high. So even if that looks smaller, the surface that can be touched can actually be larger.
It also serves as a success for your design elements. You may want a small 24×24 star to use as a clickable favorite icon. You could (and should sometimes) make a 44×44 picture with a 24×24 star in the middle.
The only exception to this rule is that your developers are well informed about the minimizable areas. Then you can create a 24×24 icon and tell them it should appear on a 44×44 loadable area.
2.3) In some cases, however, you may want, or need to, break the iOS Human Interface Guidelines (HIG). Maybe you want the small 24×24 icon and do not have space for the 44×44 area that surrounds it. It happens. But it's very important to test with users and make sure that these differences do not compromise the usability of your product.