Is the circumambulation valid if I touched an impurity in the sixth round and completed the two rounds, then purified myself immediately after the circumambulation and completed the sixth and seventh rounds that were affected by the impurity? And does anything become incumbent upon me for this after departing Makkah? And what is considered a long interval during circumambulation?

We fear that your mention of touching impurity is merely an illusion or a delusion. If that is the case, then ignore it and pay it no mind. However, assuming what you mentioned is true, then your tawaf is valid, praise be to Allah, and nothing is required of you, given the circumstances.

Firstly, because some scholars do not consider avoiding impurity obligatory during tawaf; this is the view of the Hanafis, and you may act upon it after the act has occurred and rectification would be difficult.

Secondly, because, praise be to Allah, you completed the two circuits in a state of purity, and continuous succession between the circuits of tawaf is not a condition for its validity according to the majority of scholars. From this, it becomes clear to you that your Umrah is valid, and, praise be to Allah, nothing is required of you.