1. Can a fee have a satoshi/B value below 1?
2. Can a fee have a satoshi/B value equal to 0?
3. What is the smallest size (if there is any) in bytes that a Bitcoin transaction can be?
4. Since block rewards decrease over time, is it possible to sustain a system in which miners and low on-chain fees (by low I mean less than $0.01) can coexist with a Bitcoin valued at $1 million?
1. Yes the fee of a BTC transaction can go below 1 sat per byte. Example : If you have a transaction of 200 bytes and you pay 100 satoshis for thw whole transaction that will mean that you have paid 0.5 sat per byte as the transaction fee.
2. Yes the fee can be equal to 0 but the probability for it to never be mined is very high.
3. Well I had googled it and found the best answer here :
There is no minimum size restraint on blocks and transactions. However, due to the nature of blocks and transactions, there is a practical minimum.
The smallest transaction I can think of is 61 bytes. It is a transaction that spends an OP_TRUE anyonecanspend output and creates 1 OP_TRUE anyonecanspend output. The smallest block I can think of is 146 bytes. This is the 61 byte small transaction + 4 bytes for block height in that transaction + 80 byte block header + 1 byte for transaction count.
4. According to me it might be possible that BTC will sustain the system when it reaches $1 million. Although I am weak at mathematics I think that the numbers would add up just right.