If Jackson drops below Keeper level, he still may be useful as a Ruck replacement, for when the Rucks have a Bye or a short term injury (assuming Livingstone is on your Ruck bench). And even if he drops below Keeper level, as you said, he may still be a top 6 Forward anyway.
With Heeney I'm not so sure, if he drops below Keeper scores, he might still be a top 6 Forward anyway, but I don't like his Round 5 Bye that he shares with Grundy and Daicos.
If one doesn't start with Heeney, then you won't feel foolish, as Heeney's money is being used elsewhere on a player like N. Martin, who should be making money too.