If people starded to use this P2P System, why do others people started to go outside of the original way planned by satoshi ? ( Ethereum, MtGox, Bitinstant, QuadrigaCX )
Because people can innovate, they have their own motives (the majority are for-profit obviously), some believe the network is not suitable for their purpose etc. At the end of the day, no one can guarantee that the project will always go the way its creator intended, you can even see some forks here and there claiming to follow the original idea of satoshi but some details are quite different from what has been discussed on the whitepaper.
But what bothers me is the fact that to get it in the beginning, it seems like you had to be part of PoW protocol, in order to be rewarded for your work of verification and security, in other words, to be a dev or to know one.
What is the alternative in your mind? You need to consider that the network is still in the early stages. No one would bother running software for no reason. You don't have to know the dev or be an insider, you can just download the program and run it on your own. Basically, you can mine it yourself. This way there will be competition and the distribution will be fair. At least it is fairer than producing 1 million coins out of nowhere and then sending 1 satoshi for each new user.
And that's my whole problem: how would I have held my Bitcoins, without any computer skills, without any desire to sell it, loan it or trade it, ONLY wanted to exchange my fiat in BTC, WITHOUT any third party ?
I don't get it. If you have no plan on trading Bitcoin or using it as a currency, why bother owning it in the first place? Wanting to own some BTC without learning any 'skill' or knowledge related to wallets at the very least sounds like a silly idea. It sounds like you want to ride in a car, but don't want to drive, don't want to hire somebody to drive for you, don't want to rent a car, etc. It sounds like wishful thinking if I get what you're trying to say correctly.
If this question is related to how you can get BTC without going through exchanges, as others have mentioned P2P is probably the alternative. If you don't want to interact with anyone else but get the BTC directly from the network then mining is the only choice, but it is unrealistic to do it without any capital. Hoping to mine without spending any dime is also silly since the network has grown so much compared to the early days. I believe trading or earning BTC, not through mining is still within what satoshi envisioned if that is what you're worried about. CMIIW.