We cant say if bitcoin is secure because there are no regulation about it, and there are a lot of country who banned bitcoin from their regulation, so dont use bitcoin for transaction, use bitcoin for invest, and it will help you..
Bitcoin transactions are slow and quite expensive. It's more reasonable to use it for invest and use Ethereum for transactions.
Yes, but you have to know the transaction fees of Bitcoin has reduced recently. I don't know exact the reason made the transaction fees of Bitcoin reduced, but I heard Lightning Network are used for testing to check the Bitcoin system can use this node or not. And the total number of Bitcoin transactions also decrease to transfer to altcoin transactions in the past time.
nothing to worry about the bitcoin security its more than enough to secure your capital in it and people are trusting that's why, and the journey of bitcoin is still for 9 years no such kind of issues arises in past so it will stay calm in future also.