Yes you will likely get a krausen (I use a blow off tube for all my primaries until things have settled down.)
I'm not sure what extract will give you the 'best malty flavor'...I think you could use just about any DME, but many people recommend using lighter DME; ie, if you want color and/or more roasty malt flavor, use additional specialty grains (which you could just steep in hot water...no need to actually mash) Wheat has it's own distinctive flavor, so if you really want malt flavor, you might just want to go with regular light DME. If there's a particular kind of malt that you appreciate the flavor of, I'd think about steeping a pound or so to extract some flavor and color and then add the DME to this before boiling...do you have a LHBS where you could smell/taste some of the specialty malts?
With the later additions, I wait until the really aggressive early phases of primary are finished, then add the additional fermentables. In your case, I'd heat up water, add the DME, then boil for 10-15 min. After turning off the heat, I'd add perhaps 3/4 of the maple syrup and blueberries. After cooling to pitching temps, add your yeast and let it ride. Once things start to slow down, heat up a small amount of water, and add the remaining maple and blueberries, then dump into the primary. (You may want to give a good swirl to the primary to release as much CO2 as possible...sometimes adding things like this can cause volcano eruptions....) You may get some increased fermentation activity for a couple days, then once things slow down/stop, proceed as usual