You can use the new brass (so long as we're not talking Ackley Improved here, which I don't think we are)...
You can use fired brass, or new brass either one... just keep all cases in the test as uniform (all new, all once fired, all twice fired, etc) as possible.
As to which powder you use, it's hard to say... some like the 4831, some prefer 4350... but there does seem to be a trend beginning toward H1000 and Retumbo (and presumably RL25) powders, which give a compressed load.
I use Retumbo in my .243 win... I think I was among the first to begin doing this some years back, pushing 107's to 3000 fps with it... lots of folks at the time said that powder was too slow, but results were amazing...
The 6.5-284 (with 140+ weight bullets) would probably work well with Retumbo also, but I might call Hodgdon for some info before setting starting and max loads. They do have H1000 data on their website, and that powder is very close to Retumbo--maybe a tad faster, but very close.
I'm not seeing where you've tried more than one bullet. Some bullets simply don't like some barrels. The bullet is a good portion(read most) of your accuracy so you may be chasing ghosts trying only powder swaps, etc.. I'd put a sierra or berger in it if you haven't. Are you dead set on a 140? Some rifles will do better with another weight, say 129 in your case.
Did the rifle have a bad day, or did you have the bad day? I've seen it where someone will set up with bad bagging or a bit of wobble in the bench and poor accuracy will happen too. A loose action screw or gummed up lock-work can kill a group. My 300 win will pitch 10 in an inch at 100 as fast as I can pull the trigger, but if the front screw migrates 1/2 turn out, it's to pot with accuracy. Check for things like this before you scrap your load work.
I've tried JLK's in 140 and 130. Also tried 123 AMAX.
I've also tried RL17 with all those bullets.
I'm beginning to realize that I did myself a huge disservice by starting with the "plan" that I had. I've burned a ton of money and probably 20% of my barrel life in a vicious unproductive circle.
try a sierra or hornady flat base. If the groups don't come in, the barrel probably won't give you what you want. I go to those two if the rifle is fussy. Hell, I've made my two longest kills with a 7rum and a 140 sierra fb. pro hunter. Not exactly the best bc wise. It will kill at over 700 yards; I've done it.
I haven't followed your load workup, but the biggest error most new reloaders make is to try too many variables and not do a methodical job. ...often jumping to conclusions because they had one tiny 3 shot group.
It almost sounds to me like you need to scrap everything and start over with Dan's OCW method.
I see no reason why h4831sc won't work. But, pick some known good set of components (e.g. 142 smk, lapua brass, h4831sc, cci br2 --or-- 140 berger, norma brass, h4350, fed 210m) and stick with the method.
I also might add that I rarely find best accuracy with max loads. It almost sounds like you've pre-suppossed a velocity and are trying to get accuracy at your max load.
The 140/142's are still very lethal at long range with an MV in the 2800's. If that's not ample, then you may want to consider a 7mm cartridge.
You seem confident in your shooting ability, so I would assume that technique, paralax, scope, mounts, rest, etc... are not at issue here. But, that's not always the case.
After 200+ rounds, do you have a specific load that you can say is consistently sub-MOA to 300+ yds?
With a cal. that eats it's barrel up fast I often "shotgun" the load development. I'll Take three or four bullets and pair them with the best two or three powders and shoot them with a start charge. I usually take the best one or two and work them up fully. I need to know I've got the load within 100-200 shots when I've got a sub 1000 shot barrel life.