Sashimi is good for you, and very Paleo, but 5 times a week might be excessive, because of mercury in modern seas. If we were all living 300 years in the past, then sure, eat all you want. But these days, some fish aren't as safe, and we don't have many fish left in the seas anyway. Maybe you want to add a bit of offal instead, and reduce sashimi to 2-3 times a week (in addition to other meats/fish for dinner)?
Also you should bring to the restaurant your own Tamari sauce. The normal soy sauce they give you has wheat in it. Tamari one is pure fermented soy sauce, and it's the original recipe, so it's better than the usual soy sauce.
Agree with Eugenia. The larger predatory fish, such as king mackerel, salmon, and tuna have much higher levels of mercury as well.
Also it's a good idea, I think, to avoid farmed fish. Although I love it, most sushi places I go around here (NYC) use farmed salmon...bummer. I RARELY get salmon sashimi anymore.
If you are eating fish five times a week I really don't think you need a fish oil supplement. Sashimi is theoretically healthy, but unless your restaurant uses wild caught fish (and most don't) it's not as healthy as it would have been ancestrally. Farmed fish have much, much, much lower levels of omega 3s. And, of course, the pollution levels of the oceans (and thus, the fish) are also concerns.
Have you considered sourcing wild caught fish at a local market and making sashimi yourself? It's certainly not difficult and, as an added bonus, it would be cheaper and better for you.
It's pretty good, but definitely not ideal. First, it's hard to avoid farmed fish and they can have a decent amount of omega 6. Mercury can also be a problem. Plus, it's really just a protein meal. To address the last issue, why not add something else, like rice and seaweed?
As for the second question, I would avoid fish oil supplements altogether if you're eating this much fish.
Whether or not you skip a fish oil depends on: how much sashimi you're eating, what type of fish it is, what else you're eating that day, how big your usual fish oil dose is, etc...
Tuna isn't as fatty as salmon, and if it's mostly tuna sashimi, you might as well not skip your usual fish oil.