As adiyen is staying abroad, could not perform the annual sraddha as per the norms due to a lack of facilities. Can adiyen do their sraddha during any month on their shraddham tithi when I visit India or should adiyen perform it during the exact month on their shraddha tithi? Also, adiyen could not come to India on the tithi in which they departed.

Scriptures have not given any concession to perform during different months; it is called sraddha as it must be done with faith and belief (shraddhA), and it must be done on sraddha day.
If a taint occurs on that day, one must do sraddha on the day when the taint ends. If one forgets, scriptures have stated a particular day for sraddha performance, which can never be a day as per one’s convenience.