Many questions (good ones!) and I will try to answer them all.
1. 1/4 teaspoon of k meta per 5 gallons or 1 campden tablet per gallon. If using the tablets, crush them between two spoons before adding.
2. K meta kills most microbes in solution, but slowly dissipates. It is at a low enough level 12-24 hours after dosing that it will not inhibit the pitched yeast.
3. When I get pre-frozen berries, I rarely wash them before using. I am lazy, and juice that I don't want to lose tends to seep out as the berries thaw. By not washing, you have a higher risk of contaminants getting into your mead. Contaminants could be chemical or could be spoilage microbes.
4. Wash with tap water; soap might carry over into my mead, so I avoid it.
***Onwards to the recipe critique!
Lots of recipes on other sites seem to recommend EC1118 champagne yeast. I find it to be somewhat mono-dimensional, and that it tends to blow delicate aromatics out the airlock. It does have its uses, but probably not the best choice for melomels.
Consider instead KV1116 or 71B, both made by Lalvin. KV1116 is a powerful fermenter like EC1118, but tends to yield a better mead and will maintain those more delicate aromas. 71B will also yield a very good melomel, adding a fruit salad-like character.
Recipe size is a bit larger than I normally recommend for newbees. Lot of money tied into a product that could take a turn for the worse. Consider scaling the recipe to 1 or 2 gallons until you get the hang of things.
1.7 lbs raspberries/gallon is going to give a light-medium raspberry flavor. I personally like my raspberry melomels to smack me in the face with flavor and aroma. Consider upping to 3lbs/gallon or even 4lbs/gallon.
Using the mead calculator, link is on the left, you're going to get about 13% abv with 13.5lbs honey/5 gallons. Many, if not all, of the commonly available wine yeasts will take this mead dry. Dry raspberry meads are a little harsh on the palate, so you will likely need to stabilize and back sweeten. Do a search on both as there are many posts outlining these techniques.
Oak is best measured by ounces or grams. 1.5oz of oak cubes over a month or two is generally enough for most folks. You will need to taste your mead every week after one month to ensure it is not over oaked. If you are not careful it will end up tasting like a board