1. I don't properly understand the address system it has.
When you pay a Monero address, you actually use that Monero address to generate a one time address known only to you and the recipient.
2. I can't sign transactions offline, no matter how hard I tried.
You can do this using the main Monero wallet CLI or GUI. What software are you using?
3. I am not sure whether the variable sized blocks are good or not... I haven't come to a conclusion yet, but I would like to hear your thoughts.
It's an economical debate. Bitcoin uses fixed block sizes to create a fee market to ensure security once the block subsidy is irrelevant. Monero uses a tail emission, meaning fees can be lower. Both have limits on block size to prevent centralization like we see with various Bitcoin forks with much larger blocks.