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A) It is wholly unnecessary. The smallest UTXO you will ever need is 800, and that is with a difficulty of 1.
B) Sending 10,000 GRC to 10,000 UTXO addresses will take you ages, congest the network, and cost you a small fortune in transaction fees.
C) Your wallet has to iterate over every UTXO every 8 seconds when staking. Depending on your hardware (such as if you were running a raspberry pi or other low power device) you may not be able to "check" all your UTXOs in time for each block, causing you to miss stakes.

How would we know if we were being affected by the problem described in C?

Unless you've done something silly like manually create thousands of UTXOs, that would never realistically occur. Through testing on my RPI3 I know that <500 UTXOs you see no issues at all.

Will anyone explain whether the chances to stake are exactly the same with 10K grx in 10K UTXO rather than having 1 single 10K UTXO?
Somehow, it looks like probability is not a "linear" addition of each UTXO...