Splitting a PNR does work here - your husband will end up with a separate and standalone reservation and so long a Companion Voucher wasn't involved, as soon as you are split off then his reservation is no longer affected by your actions on the original PNR. I can't see any downsides to doing this if there is a chance of the scenarios mentioned, apart from the more trivial points about making it harder to be sat next to each other.

Like the other contributors, flexible tickets usually are flexible, I'm wondering if the dates / flights used for changing are just extra expensive ones and whether you would get a better fare by going for a slightly different date. But that's not possible to work out given the information provided.