A. It’s difficult to comment as I haven’t seen the person myself. lumineers are avery thin veneers- and te under-surface colour of the natural teeth can shine through. This maty not ne the best option if you mother-in-law has yellow teeth. Porcelain veneers may be another option. the tooth that was crowned before will need another crown- if there is enough of the tooth left. this will have to be assessed aswell. It may need root filling etc.
Ideally, she would need to have a consultation to discuss her individual needs, but it sounds like a smile makeover will give her back the confidence that she lacks. Good luck.