You do not have to wait to put them in - needles are ok.

Correct on question 2.

If there is nothing but a bare pond bottom you could easily use 25 xmas trees.

Put 2 trees in water 3 ft deep within 20 ft of where you think BG may bed - lay them on their side and stake down with a stick pushed into the bottom. Use the rest in groups of 3 or 4 to a group and depending on the method used place them as follow (5 groups): 1 group in the mouth of each of the 2 major coves , 2 one group off the end of the main point about 1/3 the way across toward the dam , 3 one group out of each corner (50 ft) of the dam where it meets the side of the pond.