1 ここ
I and my husband would like to attend, but regrettably, we must decline your invitation.
は
英辞郎の下記例文が参考になると思います。なお、
My husband and I
Taro and I
のようにIは後ではないでしょうか。
ジムと私は5月2日金曜日には先約があって、残念だけど参加できないの。
Unfortunately Jim and myself have a prior engagement scheduled for Friday the 2nd May so won't be able to come.
先約があって出席できない ;《be 〜》
unable to attend due to a prior engagement
当日は先約があり、お招きいただいた来週のパーティーに伺うことができません
Previous commitments prevent me from accepting your invitation to the party next week.
2 先約を理由にすれば途中の説明は省けて
I feel really sorry that I cannot see you in wedding dress.
に飛び
We have decided that in a few years, when my daughter will be able to take care of herself, we will visit Australia to see you and your husband.
位でどうでしょうか。