As I learned (even from this forum) no one should send BTCs to the same address more than one time.
So how can it happen that there is more than 1 input per address?
Not recommended but it is fine.
The only situation I can think of is when it took place accidentally or by inexperience.
So normally (in 99%) all send addresses count as 1 input.
Am I correct?
Bitcoin doesn't work by an account system but each inputs have different conditions to be spent. Addresses are just a more simplified representation of the UTXOs associated with a certain keypair; addresses are derived using the public key.
You don't have to look at the mempool statistics if you want a general idea of how much fees you'll be paying regardless of future events or conditions. Looking at the composition of fees within the historical blocks would suffice.