@Evenstar My situation is a bit different, in that DH's kids have since gone NC with me. However...
I can tell you that there was no help whatsoever with sorting things out. The son and his partner worked from home. The daughter did have a career from which she retired early.
They were never very good at giving the kind of support that I've seen others giving to their parents (both DH and his ex, their mother). They always seemed to rely on their parents' partners or the children of a partner. (DH's ex had three partners after DH.)
DH's DIL was a bit more helpful when it came to DH's ex (which always made me wonder why she dealt with things instead of the son). One time when she couldn't deal with a hospital appointment (making me wonder why the daughter wasn't doing it) I offered to help.
No matter what they might think, Evenstar, your husband's adult children are no more bereft than you. I'm with your children - leave them to deal with it.
You could possibly give them the name of a local gardener prepared to take on the garden until it's sold.
You're telling me that they offered to help but have not followed through. I'd be given reasons (sometimes) as to why the kids wouldn't help.
When I actually asked for help advocating for DH, the son and DIL wrote a letter to the hospital doctor and copied it to me. It said that any further concerns should be referred to the son's stepmother...One of the only two times I'm aware they called me that. (The other - apparently - was when they told friends that the son's stepmother understood why he couldn't come to the funeral: he was shielding.)
When I told the daughter that DH would need care if anything happened to me, the response was "But you're all right, aren't you?" [I'd been given the all clear after a health concern.]
I've reached the conclusion that you believe what people show themselves to be and your husband's kids are unreliable.
If you're feeling charitable, you could send them links to local estate agents...but you've done enough.