A few unfinished boards are creaking on new install. Will top nailing or epoxy injections between planks stop the problem? If injection, how is it done? Please advise as to the best course of action.

If I recall, your floors were to be glued and nailed. How could any of the boards be creaky or noisy? If you have enough left over flooring, then if it were me, I would cut out the noisy boards and replace. This would allow you to determine WHY they are noisy. But if that cannot be done, then I'd try the injection route. 3M makes the epoxy, mixing nozzle and dispenser gun. You need to drill a hole large enough for the nozzle to fit in. Then simply inject thatr space with the epoxy and allow to set, about ten minutes. You then need to fix the hole. Typically, a matching filler/putty is used. Or you could install a small plug.

Ya, and have a depth gauge on your drill, or you will be filling the basement!!

I have a few of these also and I'm virtually certain it's a cracked tongue. I figured replacing them might make the situation worse long term since top nailing with straight nails would probably loosen over time (and of course you lost the bottom of the groove).

What is your technique for replacing a board so that it's as good as a board nailed with cleats?

Difficult to explain but I'll try.
1) Using a circular saw, cut two parallel lines the length of the board you are removing. The cuts should be 1/4" in from the edges and the blade set just deep enough to cut all the way through but no more than the thickness of the flooring you are removing.
2) Then connect the two cuts with a diagonal cut, starting at one end and finishing at the other. DO NOT cut past the ends of the board you are replacing, just up to it.
3) Carefully pry up the two center sections of the cut board using small pry bar or whatever. DO NOT damage the surrounding boards. The center sections should lift out easily after busting loose at the ends.
4) Now, using a chisel, carefully knock toward the center of the void, the groove side of the board and lift that out.
5) Using your chisel again, knock the tongue side toward the center as well. This will be a little harder as the fasteners may still be in the way. You may need to cut them and pry those out first. Hopefully, by cutting 1/4" in from the edge you will have cut through the fasteners.
6) Clean up sawdust and cut out area.
7) Prepare the new board by removing the bottom of the groove well in from the edge of the board. Leave the tongue alone. Cut to length exactly by cutting tongue end and cut off the bottom of groove on the grooved end. Now test fit. You will probably need to plane a back bevel on the groove side to be able to allow it to fit. Insert the tongue side first the rotate downward into place. DO NOT POUND IT HOME YET. You are just dry fitting it. Once you determine it will fit, then it will need to be glued down.
8. You a quality adhesive in a tube like PL400 or a Bostick's Best. I've even used epoxy. I'll sometimes use a little carpenter's glue in the grooves as well if it looks like that will be needed. Once the glue is spread, then install the new board by inserting the tongue into the groove and rotating downwards. You will need to tap into place with a hammer and tapping block or rubber mallet. Viola, you're done.

I lay a thin piece of fishing line across the hole before i set the board in to make sure it will fit . very easy to lift the board back out and glue .