It is good that you are curious but your many questions aren't exactly new or haven't been addressed by security experts already.
In order to answer all your questions we would need to essentially discuss all the security best practices in Bitcoin and the open source movement in general which would involve far more time than a simple thread.

Bitcoin is as secure or insecure as you choose to make it. A mutisig paper wallet with the keys stored in different locations and different forms is essentially impossible to steal, unless you torture the owner into recovering the keys and than in that case you can use a dead mans switch or ntimelock to even protect against this vector of an attack.

I would suggest you start researching into computer security by first reading the available information.

As far as getting hard numbers on how many "man hours" has been invested in Bitcoin this is impossible to obtain as Bitcoin is a global open source project in which anyone can participate and many are anonymous with their contributions.