1. Can you choose to gain fewer successes than you rolled?
2. If not, can you roll fewer dice than you are entitled to?
3. If not, can you choose to ignore the bonuses from skills?
4. From allies?
5. From items that do not require hands?
I believe so
6. Can you choose not to carry anything in your hands?
If you mean can you choose to use no weapons even though you have them than yes you can.
You cannot intentionally fail your check. What you can do is set yourself up for failure by for example lowering your skill during upkeep if you know there's a check you want to fail.
The easiest way to get hit by a night gaunt though is to minimize your sneak and "attempt" to sneak by it and auto fail by not being able to roll any dice.
You can do similar things in other situations for example if you are trying to close a gate and you want to fail but you have a good fight and a bad lore you can choose to use your lore and fail that way.