Respect or regard

Are respect and regard interchangeable in their singular form?

Are they interchangeable in their plural form?

Singular form

The union is the largest in the country and in that regard/respect is best placed to serve its members.

Plural form

This proposal differs from the last one in many important respects/regards

In most respects/regards, this film is better than the original.

Thanks

Ps is this question natural? Can you say "in their singular form"?

Are respect and regard interchangeable in their singular form?

Which is correct? In a certain respect/regard he is better than me.
In certain respects/regards he is better than me. Thank you