To me, the realization that Michael and Charity quite possibly doesn't even trigger their Bless this House in their own home kind of breaks it for me.
They both have Superb conviction (+5). Thresholds for a home start at +2, increasing by +2 for each "homely" factor, and I'd say the Carpenter home quilfies for two upgrades (children growing up there, and they are actually building an extension themselves) putting it at +6, thus cancelling out the BtH.
Personally, I'll houserule it to be a +2 bonus the a Threshold that's lower than Conviction, and +1 of the Threshold is higher (maybe with a cap of double conviction of something like that). Really, just anything to make it actually trigger in a normal family home even if you're not playing Submerged...
As to the original question, I'm with Haru here. BtH can't create a threshold from nothing, so if there's a Threshold to boost (even if it's 0) it has already blocked out any being that can't cross a Threshold.
Edit: Mixed up other posters so attributed wrongly. Corrected.