Hi DW,
I think the following have been ruled out already: Active Surveillance, HIFU, Laser Ablation. I'm only mentioning them to make sure you have a full list and everything has been considered.
In your list you have mentioned RT for clarity you should call this External Beam RT or EBRT: to distinguish it from Brachy which are still forms of RT. You should also add HT to the list, because it could be included with any of the other treatments.
As your husband already has ED, I think you will have to accept this won't be getting better on its own and he might even have to look at penile implants if he wants to. Of course telling your husband that you want to use him like a sex toy may cause issues in your relationship.
I think the definition of continent as using one or less pads per day is useless for someone who wants a normal life. Because the threshold had been set so low, any official statistics are meaningless to anyone who want to remain continent by the lay man's definition i.e. no leaks.
I think RP has the highest risk of incontinence and if you get it it would ruin your life, but it is probably not as common as this forum would lead you to believe. I think the chances of incontinence from any of the RT treatments is vanishingly small.