I don't know that I would label it foolish -- but it certainly is unnecessary! In heaven, all the sins and hurts of the past will be erased, and they'll no longer have any hold over us. In other words, in heaven your husband will be perfect -- and so will you! The Bible says that in heaven "we shall be like (Christ), for we shall see him as he is" (1 John 3:2).
In any case, you shouldn't feel guilty because you didn't carry out your husband's wishes concerning his funeral. If he felt strongly about them, he should have put them in writing and not left you to guess about them. In other words, don't be bound by undeserved guilt. When we do wrong, we should face it and seek God's forgiveness (and the forgiveness of those we may have hurt). But sometimes we feel guilty when we shouldn't, and we're only giving the devil an opportunity to weigh us down.
One reason I wanted to reprint your letter, however, is because it points out an important practical issue. It is this: We should do everything we can to avoid causing problems for those we leave behind. Even if we have very little, it's still important to have a valid will, and also documents that express our wishes concerning our healthcare. The Bible says, "Everything should be done in a fitting and orderly way" (1 Corinthians 14:40).