Q: What is the place of intellect with regard to the inner being and the outer being?
Intellectual activities are not part of the inner being, the intellect is the outer mind.
Q: When does the intellect become an obstacle to the higher realisation?
When it wants to judge things for itself instead of submitting to a higher light.
Q: Can any of the various thoughts that pass through the mind be useful to sadhana?
Not the ordinary random thoughts. If it is an idea or perception with light in it, then it can be of use. To have a developed intellect is always helpful if one can enlighten it from above and turn it to a divine use. It is only one man out of thousands who has a trained intellect. In others it is either ill-developed or undeveloped.
– Sri Aurobindo