How do you clean brass andirons?

Start by wiping off loose soot with a dry cotton cloth. You'll want to avoid using harsh modern chemicals on old brass fronts. Use a mild metal polish sparingly on the decorative finials if they look dull. Leave the heavy iron billet bars alone, as the fire will just burn off anything you apply.

Measure the depth of your firebox first. The rear billet bars need to sit comfortably inside without hitting the back wall. The front uprights should stand proud of the fire itself but remain fully on the hearthstone.

We have a very large inglenook fireplace in our living room and the iron bars we bought last year look lost in it. Can we use multiple pairs at once?

Using multiple pairs across one hearth was standard historical practice in large estate fireplaces. It supports longer logs evenly across a wider span. You just line them up parallel to each other. The air still flows underneath without any issue. This setup fills a wide inglenook visually while remaining highly functional.

The horizontal bars at the back of my ones are burnt away in the middle and sag downwards. Can a local metalworker fix this or are they ruined?

A competent blacksmith can easily weld new iron stock onto the back of an original upright. It is a standard repair for antique hearth equipment. The repair sits hidden under the logs and ash anyway. It saves the decorative fronts from being thrown away.