1 Larian has nothing to do with off line installers... they are made by Gog staff as a bonus option for Gog customers
2 Deluxe edition content comes in 2 parts, 1 installs to its own "DigitalDeluxe" folder and this has no effect on the game itself... this is stuff you can ignore or copy to a storage drive
the 2nd part is a chest with a few items to use in game and yes this chest is bugged by Larian resetting everything each pach i.e, after 3 hotfixes you will have enough cloaks to outfit your whole party because it keeps giving you a new chest of [the same] items... and yes this happens at the Larian end
as a Gog Galaxy user the deluxe edition is a Dlc so you only install it once
as a Gog off line installer user you only install it once as well so long as the main game build doesn't change i.e, these extra files a re hotfixes that do no require you to start a new save game or reinstall the base... if in the future Larian brings out a build 3 of their game then you will need to start fresh with the installs