Well BH the round ball might work .It might even save some frustration .You did not mention any ignition issues so that is a plus .
The usual things to bring it up to working condition is bed the tang and barrel, clean up the lock by adjusting the sear notch ,lightening the sear spring and polishing the bearing surfaces .Sometimes the sights can be too loose. I have used a rubber band to tighten them .All of these things will take some of the slop out of the barrel channel and the sights .
For competition and safety the trigger pull cannot be lightened more than 4 and a half pounds nor should you eliminate the half cock notch .
The question is whether you want to do all this as it does effect the resaleablilty as some people do not like a gun that has been monkeyed with .You can shim the barrel with paper or masking tape as an experiment especially under the tang to see if that brings some improvement .
Sometimes if your musket is shooting noticeably to the right that is an indicator of a really nasty trigger pull .
You mentioned 18'' groups. I am guessing you were shooting off the bench .Like any other gun that shoots a cylindro-conical bullet muskets have definite preferences .If you were developing a load for a cartridge gun you would be trying different bullets and loads . The patched round ball is much more forgiving, fewer variables.
Minie rifles that left the armory were well made and inspected for uniformity .The original barrels had progressive depth rifling, deeper groves at the breech getting shallower towards the muzzle. I understand that Parker -hale followed this rifling pattern in their repos ,Colt's repos may have in addition to the gain twist rifling they had .I know Dan Whitacre makes his custom barrels with progressive depth .He also relines barrels with original style liners . Mr Hoyt makes good minie barrels also from all accounts but uses a different rifling style . Both make replacement barrels to fit most of the replicas in addition to barrels of the original profile.
Progressive depth rifling helps true up the bullet after the skirt was blown into the deeper grooves swedging it back to a truer form .That initial upset bumps up the bullet to bore diameter ,engages the rifling and hopefully makes a good gas tight seal all at the same time .The balance of the replicas do not have progressive depth rifling from everything I have read and from the knowledgeable good shooters I have talked to .That is one of the reasons they get fussy they need just the right bullets to walk that fine line .The replicas are not true to the original system
Most reenactors shoot blanks so it is less of an issue with them and they are a significant part of the market .
Some guns that were extensively shot with blanks by CW reenactors can have excessive breech erosion which can also be a problem. Sometimes the erosion grabs part of the skirt and stays near the breech while the rest of the bullet goes on its merry crooked way.
Trash can bullets are popular as they are less fussy but are not stable out beyond a hundred yards and sometimes a bit less.The ballistically knowledgeable here could tell you why .I found for my own use the Hodgdon bullet is good to 100 without to much wind but for anything windy I go to my thin skirted Int'l Rapine.Skirt thickess is also important .If too thin it will grab the rifling ok but as it leaves the muzzle if the pressure is too high the skirt will get deformed but the escaping gases .This is a function of both powder charge ,pressure at the muzzle and barrel length .If it is too thick and does not flare into the grooves you get gas cutting and no spin .Bummer
While most every rifle ,unless the bore is not uniform ,can be tuned to shoot with the right bullet ,powder charge and granulation the search can be frustrating . Currently I shoot an extensively rebuilt Italian springfield with a progressive depth rifling in Whitacre barrel with a 1-72 twist that duplicates the armory pattern as the search frustrated me as well . I short cut much of the issue and went to a custom barrel. I had to re-bed it as the Whitacre barrel was so much thinner that the Italian barrel and I had to take up quite the barrel channel slop .I had to get different barrel bands as well as one thing lead to another .While it shoots just fine not a thing of beauty. Later I friend of mine gave me an original lock which I had Jim Westburg tune .It isn't 4 and a half pounds of pull but it is clean and safe .More acraglas fixed that too.
Last spring at the local mothers day shoot there was a guy who had just bought an dark original '63 springfield at Cabela's .The bore was rough and pitted .He too had a bunch of those lee minies he had just bought , probably the same pattern you have a mold for BH, and that rifle shot very well right to the sights first time . Some guys have all the luck .He said paid a 125 dollars for it. Like Lon Grifle I didn't have any luck with the lee traditional or improved minies either.
In the end I found it very worth while learning a great deal in the process.
You might concider trying 3f as well before you give up on that particular bullet
