From the 2 cents area, spend some time looking at a 1,2 or 3 way speakers regarding the specifications frequency, rms, etc. If looking for more bass a 1 way is going to be the option if looking for the full spectrum a 3 way may be the way to go. Individuals sound preferences are not something...
Assuming it's a typo on the 05 as the Lucerne didn't start till 06, the seats are held in with bolts #4 in the rear of the seat and slide in to slots in the front. Removing the back seat bottom will offer more room, remove headrest and don't forget to disconnect the seat to vehicle harnesses.
Wipe the bumper sensors clean and see if the problem is fixed, but as mentioned the assist module was replaced(?) it shouldn't fail this early unless it is a connection, harness or a bumper sensor issue.
The drivers door speaker is for all the chime warnings so if it's shorting out no warnings. Check the door harness for damage or cracks between the door and frame inside the bellows as I had a hairline in mine and it was intermittent until it wasn't. But if you have the original speakers they...
Check crutchfield.com, but there doesn't appear to have a dash kit available plus getting a head unit off ebay if problems occur will be a nightmare to resolve.
Can't confirm on your vehicle or year but swapping the VCIM with Bluetooth VCIM if that is all you are looking for may be a option to...
Check this site for new motor and either the PA or Lesabre forum there is a full write up on replacement, hopefully someone else on the forum can remember the post.
Lots of information on this subject, pros and cons, but enough to at least look at a different size screen as a option vs removal.