Does 'Umrah expiate grave sins like adultery, theft, and usury? And is there a way for repentance for someone who frequently commits sins and returns to them after repenting?

The door of repentance is open for a Muslim as long as he has not gurgled [i.e., death throes] or the sun has not risen from the West. Your repeated return to sin does not prevent your repentance. Allah's mercy is vast, but it should not be taken as a means to encourage committing sins. Umrah does not expiate major sins like adultery, theft, and usury, except through repentance from them. This is because the five daily prayers — which are greater than Hajj and Umrah — do not expiate minor sins except by avoiding major ones.