I think you're being unreasonable. Your husband's grandma died only a couple of weeks ago, his parents are possibly being unreasonable re. inheritance and you're trying to make it about you (being cut out and excluded from the decision, when it seems it's not even been your husband's decision anyway).
You said your husband has been 'ground down' by his grandma's long illness, I imagine the last thing he wants right now is an argument with either his parents or you about money. He's probably emotionally exhausted.
What would his Grandma have wanted him to do with the money? Are his parents actually just trying to ensure her wishes are carried out - whether or not they're legally over-reaching (which depends on the actual set up of the will)? Or would his Grandma have really wanted him to have some fun with (some of) the money?
For me if I were to inherit some money it would be really important to me to use it in a fashion which respected the wishes of the person who'd left it to me - whether they were someone who really wanted to give me some financial stability, or someone who wanted me to do something fun.
Either way, he's not suggesting something massively irresponsible, but something which long-term will benefit your whole family. I think it's a sensible decision. Even if you and he ultimately decide you want to use some of it for a treat - there's no rush to do that, let him grieve for his Grandma first and then re-open the question, when he's in a position emotionally to actually enjoy it and get joy from her memory rather than when he's trying to recover. Practically speaking you won't be able to lock away the full 10k in a LISA immediately anyway, so there's no rush to make the decision. I think you should leave the topic well alone for now and focus on giving him whatever practical or emotional support he needs to deal with his parents and the loss of his Grandma. Given that you've said you normally have a good, equal, relationship, I think if he is being at all unreasonable now, it's probably to do with the stress of the whole situation and given a bit of time and space he'll be more open to discussing it with you.
It sounds to me more like it's an issue with your in-laws rather than with your husband. But when they and your husband are so recently bereaved, now's not really the time to make a fuss about that.