There is nothing wrong with making a fabricated crank. Personally like I stated earlier I prefer making the throws from some type of steel. I have found that the reason for warping after soldering is that one side of the crank gets soldered first then the other. When the through stock from the shaft is cut from between the webs sometimes and I say, sometimes, the crank will warp. One way to help prevent this is to solder both sides and then bring them both up to the melting temperature of the solder. Except for the largest crankshafts this can be done with 2 propane torches. What this does is allow the solder on both webs to harden at the same time and generally this will prevent warpage. If when removing the center material some warping is detected it's generally always in line with the shafts, hardly ever is there a twist in the crank. To remedy this I put the crank in the lathe between centers and check it with a dial indicator, noting the high spot. If the high spot is toward the throw, main shaft area is narrow, then take 2 wedges like 2 morse taper shank removal tools. and slide them into the crank area to spread the webs. If the high spot is toward the main shaft then use a small clamp on the webs and squeeze them together. Either way at first apply a reasonable amount of pressure and then reindicate. If it hasn't moved then you will know how much pressure to apply the second time. The material will generally flex and spring back so enough pressure has to be applied to get it to move.
As far as removing the flux all that needs to be done is to boil some water and insert the soldered piece. After a couple of minutes all the flux will have been removed. There is no need for acids and chipping. If you have never tried it then please do.
gbritnell