In Skyrim, how can I keep the companion after getting married

Here is what you need to know about Draugr Wight Overlord:

• He uses the disarm shout. Its really annoying when he does so, especially against Lydia as NPCs typically don't go and pick up their weapons and will end up standing there. Try giving her a nice staff and/or bow to use so she's out of the shout range instead.
• He's immune to poison and resists frost, so stick with fire or lightning.
• They hurt a lot up close with their enchanted weapons, and they have high armor.

Here are some typical strategies to use:

• Take advantage of the traps in the dungeon. If there are any traps nearby, lead the draugr to it and have it hit them repeatedly. You mentioned the fireball traps, position yourself so the lord is between you and the crystal.
• Take advantage of your own shouts, you can buy yourself some time by using just the FUS part of unrelenting force to stagger quite often (even better if you have the amulet of Talos and Talos' blessing). Throwing in the bash from your shield to stagger them even more.
• Find some open spacing. Other than the shout, the wight lords have no ranged attack, prick them to death with arrows. (This is harder to do if you intend on saving Lydia as she'll most likely die).
• Give Lydia ranged attacks and just kite the lord around while keeping his attention.

If all else fails... turn him into a sweet roll

Similar to this answer, as long as it's not a unique item you might be able to get away with the following:

• Find the item elsewhere
• Go to Lydia's body, "store" the item and then close the inventory
• Retrieve the item from her body.