You can't really compare prices that way from Amazon. I often see stuff on Amazon that is much more expensive than what is available locally. Often, it is the exact same thing, but you are paying extra for the convenience of ordering from the comfort of your home. So, I don't assume that something on Amazon is better quality because it costs more than the "cheap stuff". It might be, but you can't judge that by an Amazon price.
Oh dear. Miso paste definitely needs dashi in your stock, no question. It absolutely changes the flavour to the stock that I think you might want to taste.
Dash of fish sauce, a bit of palm sugar and some lime juice and you’re golden.
I buy the brand that I like best for all the above ingredients, but I certainly wouldn’t skimp on quality miso paste or fish sauce.
I’m not sure about what you mean about splattering when you eat it, but a Chinese soup spoon is designed to be used with broths like this.
And I cook the noodles separately and add them just at the end to warm.