As to the weapons, this should work (i created a few NEW weapons but the replacement process is very simillar)

1) Add your new weapon mesh to Resourse manager
*nothing special here, as ussual, create folder / package and "add resourse" - model

2) Find a weapon you want to replace in RootTemplate
*will be in "weapons" folder.
*optional - to be sure it is a right one, better to follow it the whole way from TreasureTable> stats/weapon > ItemProgressionVisual > RootTemplate
3) Copy it's GUID numbers (can be done with RMB -> copy), save it somewhere

4) RMB "create new from existent"
5) Change the VisualResourse (the model you created and added to ResourseManager) and file name (won't let you save with same name) and press "create"

6) Now find your created weapon in RootTemplate ("all" folder + search tab, will be blue colored)
7) Copy the GUID number of your new weapon and save it somewhere

8) Copy a file ItemProgressionVisuals.txt from (Data\Public\Shared\Stats\Generated\Data) to
*optional. Same can be done with editor "ItemProgression" tab (just copy all and paste in your mod), but it is ussually so buggy that i prefere using notepad directly.

9) Open it, find and replace the GUID values of the weapon you are replacing to the numbers of your created weapon. (the numbers you "saved somewhere")
*there should be a few of them, not just 1, use search

This file is responceble for generating items, can tweak lvl it will drop for, can replace a few other meshes if you want etc.

Hope it helped.