Learn Farsi: به هر حال، ما باید به این مطلب برگردیم.

The phrase be har hâl literally translates to "in any case" or "to any condition." As a discourse marker, you use it just like "anyway" in English to dismiss whatever was just said (such as a tangent or a minor objection) and firmly resume the main topic. It tells the listener that regardless of the previous point, you are moving back to the core issue.

Could I use dar har surat instead of be har hâl in this sentence?

Yes, they are highly interchangeable in this context. Both are used to say "at any rate" or "in any case" to brush aside a preceding point. be har hâl is slightly more common in everyday spoken Farsi when cutting a tangent short, while dar har surat (literally "in any form") can sound just a tiny bit more formal, but both work perfectly here.

Why doesn't the subjunctive verb bargardim have the usual be- prefix after bâyad?

The verb bargashtan (to return) is formed with the directional prefix bar- attached to the base verb. When a Farsi verb already has a built-in prefix like bar-, dar-, or farâ-, the usual subjunctive be- prefix is dropped. So, to fulfill the subjunctive mood required by the modal verb bâyad (must), we just use the present stem and ending to get bargardim instead of stacking another prefix.

What kind of "matter" does matlab refer to here?