Use 100amp breaker as main on 200amp MLO panel

Is this a question?

I will say, if you backfeed a panel through a branch circuit breaker the breaker must be tied down. Most panel makers offer kits specifically for this.

Stan, do you mean bolted down?

Whatever is appropriate for the panel.
Bolted, clipped, screwed, whatever.

For the standard plugin type breaker used for residential, your looking for a Main Breaker Retainer Kit.

What do you have for a panel, I can give you the part # for the retainer clip. Also, you can't use the 2 spots across the main.