FLR(M) - online application vs biometrics appointment

Back to this wonderful resource just to make sure I'm not going to get my daughter-in-law in trouble!

She had her A2 English Test today and passed and was told she should get her certificate next week. I'm going over all their docs with them this weekend to make sure they have everything they need (did my husband's in November so hoping to point them in the right direction)

Am I right in thinking it's NOT a huge problem if they submit their online application this weekend but can't get a biometrics appointment until after her visa actually expires on March 6? As long as the online application is submitted, they can upload their docs and wait for the first available appointment?

Thanks for your help, as always!

You are correct.  As long as her online application is submitted in time, the old visa is extended until a decision is made, even if she isn't able to get a biometrics appointment for weeks.  But be sure her English test certificate, and all other supporting documents, have arrived before she applies.  Saying "It's in the post" isn't as certain as having it physically in hand.