Regarding your question about the Khuffayn falling to the heel during the Tawaf of Umrah not caused by one’s own intent. Firstly, we need to understand that the Ulama have mentioned:
أنه إن أخرج أكثر العقب من الخف انتقض مسحه وإلا فلا
(If the Khuff exceeds the heel then the Masah is broken excluding that it doesn’t break)
Now we recognize that if a person takes his Khuff off beyond the heel that his Masah is broken and he will have to wash both feet however if the Khuff doesn’t fall beyond heel then his Wudhu and Masah is still intact. Now factor in the added condition of it being one own intent we see that multiple Ulama have mentioned:
بما إذا كان بنية نزع الخف؛ أما إذا لم يكن: أي زوال عقبه بنيته بل لسعة أو غيرها فلا ينقض بالإجماع
(When the person has the intention of removing the Khuff beyond the heel then definitely his Masah will break, however if the person does not intend for his Khuff to be removed then the Masah will remain intact.)
In conclusion after analyzing all these factors we can understand that the person whose Khuff falls to the heel without his own intent that the Masah is intact and by extent the Tawaf of Umrah will not be affected.
Allah Knows Best
Zain Al-Abidin
Walaykum Salaam