At the beggining the vast majority was mining nodes . It was the only time that bitcoin was decentralised by your definition , thousands for nodes .
No, that's according to your definition. If miners have all the power, then if everyone was a miner, it'd be completely decentralized. That's yours. How many blocks with fake transactions have you find ? If it's at least one than yes , i'm wrong and you're right .
I'm sure that, whether I've found fake transactions or not, my current state is the true one. My node has synced, and verified everything since the genesis block. If I was to listen to SPV nodes, as you suggest, I couldn't know if I had fake transactions. So , by you logic , why the merkle tree is needed ?
To make verification more efficient, and to allow SPV. As for the arguments of the post you shared , these are complete BS . Initial blocksize was 32 MB and there was a reason at the time that 1MB was imposed .
Completely ignoring all the points and simply sticking with the 32 MB block size. Nice. 
By decision making i mean what transactions will be added into a block and keep building the chain .
The order of the transactions isn't the sole process of the Bitcoin network. To achieve a consensus in pool level the majority of honest pools decide if a block is valid or invalid , following the rules .
No, they don't. I don't know who told you that miners decide the rules, they only decide the order of the transactions, for if they did, we'd already be having 1 GB blocks.