You won't like the results of trying to substitute WW for plumbers lead. The WW expands slightly after cooling, -along with being substantially harder than plumbers lead. (Yes, plumbers lead has a trace amount of tin and whatever else was in the ore/slag/ect. hence not PURE lead). The result is that they are somewhat harder to load (you'll probably need a leather mallet or plastic hammer to load bullets).
I've had decent results with the Lee REAl bullet, and the Minie' may be 'ok'. I've had better results with WW and round ball, as you can go to a thinner patch material and compensate for the difference in diameters. (.002-.003"). Something I've been thinking of trying is to anneal the WW after casting it, to see if that makes it any easier to seat the bullets.
There is really no worry about pressures from M-L, as the bullet is already swagged to bore diameter by the seating process. Actually, the opposite problem is that with greater hardness, you may not get obturation of the base of the bullet and will get gas escaping around the heel and subsequently poor velocity, accuracy, and leading, much like occurs with light loads and hard cast pistol bullets.
I've also had similar experience with trying to use shotshell shot. If you really want to use the WW in your muzzleloader, the best route is to cast them into a pistol bullet and shoot them in a Sabot. There is no problem associated with the hardness and the bullets can be sized to proper diameter. I suggest you use a .45 acp bullet in .50 cal. and use 185gr to 200gr bullets unless you have a twist faster than 1-48". I've had good luck with the Lee 200gr RF, sized (not lubed) to .452, and fired in Hornady Sabot through a CVA-Inline w/1-28" twist). Still MUCH cheaper than most of the "pre-packaged" rounds.