1). As I understand it, the 10 minutes delay is to create a new block that there's enough/ sufficient time to fill and confirm the transactions (globally). But isn't it quite wasteful to keep adjusting the difficulty level to maintain the 10 minutes delay ? What is so special about the 10 minutes delay? Has there been any study to quantify more accurately the amount of time to reach the nodes globally ?
You're not going to get the network any more energy efficient -- ie. higher transaction throughput for less electricity spent -- by simply decreasing the block interval. Apart from the orphan / propagation problem that aleksej already mentioned, this would also cause the blockchain to grow faster than is likely sustainable for healthy node decentralization.
2). Let say if we don't have to worry about the trust issue, in theory we only need one, or perhaps two confirmations right ? So for the remaining 51% confirmations is essentially to ensure that there's no cheating could happen. I got that so far...
Yes. In practice you can very well accept small transactions with just one confirmation. If it weren't for a full mempool, even zero confirmations would be fine for some cases.
3). But what if we could accomplish item#1 and #2 in 5 minutes, 3 minutes or even 30 seconds - shouldn't that be a good thing ? Why make it more difficult, delay until 10 minutes and empowering expensive equipment ? That seems to feed into the endless cycles of who's got bigger and faster mining capability? Understand that it seems to justify the rise in bitcoin price...
Hashrate will always be a reflection of Bitcoin's price and the size of its block rewards / transaction fees. As long as its profitable to squeeze some more miners in, that's exactly what's going to happen. Block intervals have little to do with mining profitability and reducing block intervals would
not cause a decrease in hashrate.
All things equal, the only way to reduce electricity spent on Bitcoin is by reducing the block reward. Which is exactly what is happening every ~4 years. It's just that Bitcoin's price and thus mining profitability has always managed to keep up with the decline of mining profitability caused by the halvings -- and then some.
4). What about making the 51st % node (with the longest chain) to be a lucky lottery winner and avoid increasing the difficulty level and eliminate the need for fancy equipment and energy burning all together -wouldn't that be more fair and efficient ways to utilize energy and resources ? At the same time avoiding the centralization of miners all together ??
I don't understand what you are trying to suggest. What, according to your definition, would be the 51st % node? Every node follows the longest chain. Every miner that creates a new block by definition has prolonged the longest chain and thus created the longest chain.