Routing traces over traces in two layer pcb
The points that you have highlighted are all valid, but don't really apply to your design, as a 8MHz microcontroller is not really considered high speed.
In terms of EMI, what you should look at is rise time, rather than clock frequency, as the 8MHz frequency itself doesn't really cause issues, but a 1Hz signal with a fast rise time, can cause havoc.

To answer your questions:


It is good practice to avoid crossing other signals, but the reason for having a 2 layer board is mostly because you need signals to cross. In general terms, you should only avoid crossing signals that can interact with each other, for example keep well away from each other an SPI clock from an ADC input, if these are likely to be working at the same time, but if these are active at different times, then the issue doesn't exist. In essence, be pragmatic.
Power traces can cross over each other without any issue, as long as they are not noisy. For example, a power line that drives a motor should not cross over the reference voltage of an opamp. Again, be pragmatic.
Daisy chaining is not a problem, as long as you keep into consideration what is powered by this line. The width is proportional to the length and the amount of current that goes through this trace. As a rule of thumb, go as wide as you can, but don't forget that the return path should be at least just as wide.
Only on hi-speed designs. Don't worry about that on your design.


As you can see, there is no golden rule, PCB design is mostly about compromises. You could follow all of the best practices for signal integrity, and you would end up with a very expensive multilayer board that is space-grade, but you need to ask yourself the question: "is that what I need?"

The first step in the design is to look at the recommended layout in the datasheet. If you follow that, you should be ok in 99% of cases, especially in the buck/boost converter.
The design you are working on sounds very much like it is very similar to an Arduino board. you can download the design files one any of them, and have a look at the layout. That will give you a