Taking an oath that one will become a kaafir

Q: I made an oath that I won’t do a particular thing in my life ever again at any time in the future. If I do, I will become a kaafir. In the Tamil language any time gives the meaning of whenever, anytime in the future also used instead of whenever. I don’t know exact what I meant. Some times I feel like I might have used whenever word also. Now how can I be free from this oath. Does that mean I will become kaafir whenever I doit ?

Performing two rakaats sunnat before zuhr due to the fardh salaah commencing

Q: I was offering the four rakat sunnat before zuhr in the mosque. I was offering the second rakat when the iqaamat started, so I completed my salah like a two rakah prayer to join the jamaat. Will I have to offer the sunnat again after zuhr or is it permissible to skip it? And also should I do sajdah sahw when I repeat the sunnah to after the jamaat?