Fish – How should Dashi made with kelp and bonito flakes smell

First, about the temperature. Your safest option is to use a gun ;) The correct temp for shallow frying is between 150 and 190 degrees celsius. So if you have an infrared "gun" (a thermometer which neasures the temp of the surface at which it is pointed), use it to determine the stove setting at which the temperature of the dry (not ptfe coated) pan stabilizes in this interval after longer heating. If you don't have it, use the typical tricks for determining the pan temp, they are described in other questions here on SA.

Important: different stoves heat to different temperatures. Don't let the numbers at the knob to fool it into thinking that a given temp is "medium high" because it has the number 7. My own stove stabilizes at ~180 degrees C at setting 2 (out of 10), with setting 3 pushing 230. On the other hand, the minimum temperature for cooking fish is 50 deg. As stoves rarely go below 80 degrees, there is practically no chance to set your stove to a temperature so low that the fish stays undercooked (but it might need unreasonably long times on very low settings).

As for the spraying: the fat at frying temperature will always spray. You can still reduce your problems. First, the bubble explosions at the lower temperature will be less violent. Second, surface moisture promotes spraying (which is produced by water being instantly turned into steam in the oil). So pat the fish dry with paper before frying it. If this isn't enough, you can also drag it through flour. This will make the crust crisper. As the spraying won't be eliminated completely, don't throw in the pieces. Take a flat spatula with a long handle, place the piece on the working end, step back from the stove and slowly lay the fish into the oil, gradually pulling the spatula from beneath it. The turning should produce less spraying, so using a fork or spoon to press the fish against the spatula isn't a problem (or kitchen pincers, if you have those).

All of the above applies to everything you are shallow frying, not only fish. Only the flour is unsuitable for some vegetables.

Dashi is almost never used without adding additional seasoning, generally some combination of mirin and/or sake, salt, and soy sauce or miso, and often a small amount of sugar. If your flavor isn't strong enough, most likely your culprit is inadequate salt.

I never make dashi without using konbu, but I'm vegetarian, so mine is typically based on konbu and dried porcini, or just konbu alone for certain applications. I'd probably only consider skipping konbu if I was making a dashi based on niboshi, also called iriko (dried sardines), as is common in Kyushu and some other regions. Iriko are pretty aromatic and apparently contain enough glutamates that at least some people skip konbu.

It's not unheard of to leave out konbu in katsuodashi, but unless you're using thick or freshly shaved katsuobushi, you'll end up with a basically bitter, uncomplex version that's mostly suitable for use in dishes with lots of ingredients. I might use a konbu-less dashi in a dish that has cabbage in it, because cabbage provides its own source of glutamate proteins.

Dashi is the analog of stock, not the analog of broth. Stock typically contains minimal to no salt, and therefore the primary contribution is aroma, which is only perceived as flavor once you add something salty to it. Once you add seasonings, dashi becomes "kakejiru" if it's at the right saltiness level for noodles. If you make it saltier it's a "tsuyu" or dipping sauce.

If you're going to cook Japanese food, you'll need to get over any issues you have with salt. Actually, if you're going to be making any kind of soups, I'd say the same thing regardless of cuisine. But Japanese food uses salt, or salty ingredients like miso and soy sauce, in relatively large quantities.

Usually, your main dish won't be incredibly salty (except sometimes at breakfast), but most of the sides and soups are fairly high in salt content. The source of the salt varies, as there are two common salty ingredients (miso and soy sauce), and regional things like shottsuru (Akita's fish sauce); wet sea salt is used to adjust seasoning, even when fermented salty ingredients are present in a dish. Most meals include some fairly salty pickled vegetables; the rice balances out the rest of the salt.

One of my Japanese friends lamented that her husband's salt-sensitive hypertension was a big problem for her because she can cook low fat, or low salt, but not both, and low-salt was a much bigger problem for Japanese cuisine, as even high-fat dishes tend to have a fair amount of salt.