Preparing for third mead - more questions! :)

If you're fermenting fruit, using a bucket is a huge help. You'll want to manage the cap (you can search for "cap management" for lots of details there), which involves pushing it down into the liquid frequently. A bucket with a large opening makes this a lot easier.

Also, you can look into the foam control drops. Medsen swears by them.

For your questions, yes you can do 2 gallon primary in a 3 gallon container. Heck, you can use a 100 gallon container if you want. (ok, that would be a little silly) Headspace is not a problem during vigorous fermentation, unless you don't have enough!

When to add fruit is the $64,000 question. It really depends on what you want. The earlier you add the fruit, the more fermented character the fruit flavor will have. Think wine vs. grapes. Also some of the aroma compounds can be blown off during vigorous fermentation, so if you've got a really delicate fruit aroma (peach, watermelon, etc) then you're going to either add so much that the aroma loss isn't a big deal or add the fruit later. Raspberries usually have a pretty potent aroma, so this isn't a large concern here. Late fruit additions will give you a more "fresh fruit" flavor.

So...you just need to decide where to go given all that information. Very often folks will add some fruit in primary and then add more afterward to boost the fruit flavor. Keep adding fruit until you're happy. You might not want to add any, or you might find yourself with a trunkload of berries.

On that note, you should probably get more berries. ;D

71b is a good yeast for most fruits, it will accentuate the fresh fruit flavors and is often described as having a "fruit salad" profile.

I think D47 would be good for raspberries too, but it will give you a totally different mead. For starters I would say go with the 71B, but if you have the opportunity try out the D47 sometime and see what it gives you.

I'm currently doing a a yeast test with 71B and D47 in a raspberry melomel; so far the results for both have been good but i'm erring toward the 71B.
They're both about 3 weeks in, primary using fermenting buckets completed in under a week for both, but 71B has been slightly quicker in each aspect (primary ferment, settling in secondary, and in particular degassing). They both smell amazing, and I expect they will each have excellent (yet different) flavour profiles so you can't really go wrong with either.