Aid; assist

Hi,

What is the difference between 'assist' and 'aid'?

We'll do all we can to assist you.
We will assist you in finding somewhere to live.
The play was directed by Mike Johnson, assisted by Sharon Gale.

Two men are assisting the police with their enquiries.

a course to assist adults to return to the labour market

Anyone willing to assist can contact this number.

We are looking for people who would be willing to assist in the group's work.
activities that will assist the decision-making process

The new test should aid in the early detection of the disease.
This feature is designed to aid inexperienced users.

They were accused of aiding his escape.

They were accused of aiding him in his escape.
Aided by heat and strong winds, the fire quickly spread.

Can you please help me identify in which of the above sentences are 'aid' and 'assist' interchangeable?

Can I remove 'in' from the sentence in red?

Thanks in advance