Husbands PIP review

Hello I hope someone can help with my question.
My husband had a tree year award of low rate care, back in July he was asked to fill out a review form which he did. We used the original claim answers and added some new details along with medical evidence. Claim pack was returned sent first class recorded tracked and signed for ( I don't trust their own pre paid envelope) we had a text confirming they had received pack.
Husband phoned PIP December to see if there was any update on how the review was progressing, to be told it had not been looked at yet. So give it another month if not heard anything call again.
We are aware there is a lengthy wait for reviews, we decided to wait until beginning of February to call again. This time my husband told them he was concerned that we had heard nothing from them apart from initial txt saying they had got his review pack, and we are concerned as the claim ends on 7th April 2024, could they tell us what would happen if the review was not completed by then? we were told the claim would be extended by 12 months and not to worry. Husband asked for a letter to confirm the extension and was told DWP are no longer sending extension letters out to claimants as the 12 month extension could be less or more.....!!
Husband explained we are in receipt of legacy benefit Income support how would this effect that..?
The person on the phone had no answers for that.
My question is as off 7th April when husbands award officially ends and we are given an extension will Income support remain in place? or will we be forced onto UC rather then wait and keep SDP.. Also is there any way to get an extension letter out of them.?
Many thanks in advance.

Many thanks for this. I will definitely send a letter off tomorrow.
Very stressful situation.☹️