Helpful.
How do I properly clean the rifle?
There are as many different 'proper' ways to clean a black powder rifle are there are people doing the cleaning. Everyone has his 'my way works best' formula. I use hot water with soap, followed by hot rinse water and thorough drying with patches and towels. Run a basin/pail/bucket/sink 'full' of hot water with soap. Remove the barrel from the rifle (some folks don't do this - the following can be done with an adapter screwed into the nipple hole and a length of hose with the end submerged in the soapy water) and place the breech end in the water - the nipple hole needs to be under water. Place a wetted patch over the muzzle end and run it down the bore with a rod with a cleaning jag on the end. When it reaches the bottom it will force dirty water out the nipple hole. Draw the rod back out and it will suction water up into the bore, cleaning it out as you go. Repeat several times until the water that comes out is clear. Drain the basin/pail/bucket/sink and fill with clear hot water. Repeat the patch/rod routine to rinse. Dry with several dry patches as soon as possible. Clean the nipple by soaking in hot water/soap; use a wire pick to clear the touch hole and a pipe cleaner from the other end. Use a soft brush and a little water/soap around the nipple hole. Dry both the nipple hole and the nipple thoroughly. Oil all parts, bore, etc. with non-petroleum based oil - I'm using Ballistol but there are several adequate commercial products and some home-brewed solutions that will work. Use a good anti-seize grease on the nipple threads.
What is the optimum powder load and type, and will it safely and accurately fire a conical (Minie Ball type) projectile?
Impossible to answer. Each gun and gun owner likes their own solution. I'm a patched round ball fan; for a .50 I'd start with 70 gr ffg real black powder and a .490 ball with a 0.015 lubed patch. But there are many good combinations that will work also. Try 80 or 90 grains. Try fffg real black or Pyrodex or 777. Try .018 thick patches. Vary the patch lube material (spit, bore butter, moose snot, Ballistol/water solution,...) This can go on and on and only you can decide what works best for your gun. It's a great excuse to get out and pull the trigger. This sport is not for a lazy man.
PS: yes, it will safely fire a minie ball/conical.