Hello I have actually just recently done exactly what your trying to do so maybe I could help. I’m not sure what your were having problems with but I can start with saying what I did. I started with pipes same as you. I first cut them into smaller chucks and clean them. I cut off any piece that had solder or anything simply because I didn’t like the idea of lead or anything to contaminateing the copper. Then when I melt it down I have to borrow a friends propane forge because the coal forge has a hard time heating up a large amount to melting temperature. After pouring into a long bar shaped mold I made I forged it like a would forge a piece of iron. I dont see any reason not to forge it hot as long as your mindful of its melting temperature. The picture shows some of the stages I did it in raw pipes, chopped up smaller, washed and sorted free of solder/brazing, poured bars, and a forged leaf.
as for “deoxidizing” the copper I never did anything like that and I was also surprised that there was absolutely zero slag on the surface before pouring. I’m not sure what I did right to have it work out so clean maybe because I cleaned it a lot in advance I don’t know.