I have also seen the situation where a weapon shows a different level when equipped versus the leve it shows while in inventory.
The maximum number of Underdome bank slots is 42 (level 10). This is available once your character reaches level 48. See the Borderlands wiki for details,
http://borderlands.wikia.com/wiki/Bank.
On the second play-through of the main game, you only have a
chance to get the backpack SDU reward on the first 5 clap-trap rescue missions. See
http://borderlands.wikia.com/wiki/Backpack.
Tip 1: Get each mission and the toolkit but do not turn in. Next complete one or more other missions that give a reward item but do not turn in. Now, fill you inventory to the maximum (so you can carry no more). Next
turn in some other mission to get a reward - your backpack is now overloaded (like 43 of 42) and be sure to get a game save at this point. Now turn in the claptrap mission.
Do not exit the mission turn-in screen. If the reward is the backpack, you are good. If not, hit the XBox button to get to the XBox menu then exit to dashboard. Any other action, such as exiting the mission turn-in screen, will complete the mission.
Assuming you exit to dashboard cleanly, you can try again (mission turn-in). Repeat until backpack is rewarded.
Tip 2: Being more overloaded may increase your chance. One way to handle this is to start the first five clap-trap rescue mission (and getting the toolkit) but deferring. Then later, use the first tip to get the first
SDU (order does not matter). Do not apply the SDU. You backpack will now be +2 overloaded. Do the next turn-in the same way; now at +3. Once you have all five, you can now apply them.
Tip 3: Be prepare for may attempts at each turn-in. I just did a run where it took an average of 7 attempts per clap-trap. On an earlier character, the Sledge Safehouse mission took 20-30 attempts to get the backpack.
In regards to artifacts, the highest level is 6 (level 44 skill requirement). If you are seeing level 5 or 6, you are using your highest/last applied artifact. See
http://borderlands.wikia.com/wiki/Artifact.