Question 1: Answer can be found using SEARCH and reading the FAQ: Maybe this will get you started.
How do I edit the "Sent from my BlackBerry®..." message? - BlackBerryFAQ
Question 2: might be a memory problem as BB's begin to delete email, phone logs etc. when memory gets low. How many apps do you have loaded? Are all your ringtones, music, photos and the SD? How much free memory do you have?
Keep in mind that all messages are stored in the main messages folder and a folder for each account. Delete from one folder, it is deleted from all. Could that be the problem?