Confused with Tenses

Scenario (#1 & 2): I just drop a piece of paper and I will pick it up in a minute when I get a pen. A lady walks by and picks it up before I do. I say to her:

I still haven't gotten my pen yet, so which one should I use?
1. I was going to pick it up when I get a pen.
2. I was going to pick it up when I got a pen.

What do these mean?
3. Lucky I didn't throw this thing at you. You would't have caught it.
4. Lucky I hadn't throw this thing at you. You would't have caught it. ( I don't need to use 'hadn't ' here right? I have a conditional in the second sentence, does it connect with the first sentence?)

Scenario (#5 & 6): I find something that was used long time ago and now it's replaced by the new doors and we are using it.

Do I use 'had' or 'have' here?
5. I think we had used the alarms before we had the new doors.
6. I think we had used the alarms before we have the new doors. (If 'have' here is wrong, why? We are using the new doors right now? It is not correct to use 'have'?)

thanks.