Any cracking around the web of the brass is cause for concern and those cases should be culled. Full length resizing will cause your brass to lengthen more than does neck sizing, how ever for use in a hunting rifle I prefer to full length resize to ensure easy chambering. When a belted case is full length resized it will head space on the belt, however neck sizing or partial FL resizing will allow the case to headspace on the shoulder. The reason your case stretches is that each time you fire, the case will stretch until the shoulder comes in contact with the chamber wall, the more the shoulder is set back, the more the case will stretch. This is the reason that neck sizing will produce longer case life than FL resizing.
If you shoot a steady diet of full powered loads, the primer pockets will probably open up before case failure from stretching becomes an issue. However I'd be vigilant for the type of cracking you are observing around the belt, particularly if there are carbon traces present along those cracks. This is a warning of incipient case failure, meaning that within the next reload or two you could have a case head separation. Should that happen you might find it difficult to remove the body of the brass case without any head for a stuck case remover to grab. If you are unable to remove the case with a jag and doubled up patches, you can carefully cut threads on the inside of the case body with a tap and thread in a bolt with a stack of washers as spacers to pull it out; but its better to avoid all that by not using brass that's about to fail.
You'll never get the rifle to shoot well by shooting a variety of ammo in it. To solve your accuracy issues, work up to the maximum load, then shoot groups with a incrementally smaller powder charges until you hit the sweet spot. You will have to repeat this exercise any time you make a change in the components you are loading. If tweaking the load doesn't tighten up the group, I'd look at the rifle, particularly the bedding. This is all on the assumption that you have taken care to ensure the action bolts and the scope mounting hardware is all tight.
