1. You will not lose the dlc you purchase.
2. The dlc is currently being wiped out. I've heard the EU dlc WKC2 items are going away and some items are now free such as geonet license and guild concierge. Why have dlc items for a game that no longer supports online when it was centered around online? DLC items will go away.
3. If you play DLC quests before servers go down, you will still have access to those quests. However, once the online is shut off and the DLC items are removed, you will not have any access to DLC content.
4. I never did platinum for this game because trophies are meaningless to me. Someone else with more experience might add to this. There are trophies such as complete 50 quests S rank and bind hundreds of items. It will be impossible to beat some quests without a team (Rival Survival 2, Abyss 2, Enemy Mine 2, etc.) So you will not be able to platinum the game (without modding for superstats) if you don't have enough team quests done before the server goes down.
I played over 1300 hours and got burnt out but play on weekends some now. My recommendation is join a guild or find friends that have similar interests as you and play at the same time as you.
Should also be mentioned that some people have had issues with trophies not appearing.
The S ranking quests & 800 binds trophies are the most common (from what Ive heard).