I think that umbrella funds are the way to go. Directly funding initiatives with the SPS are hard to plan for. If an umbrella fund only achieves 50% of the required funding, they could stipulate ahead of time that, in such a situation, they'll cancel the event and a) roll the funds over to the next scheduled event or b) just return the funds back to the SPS.
I agree that with the current state of SPS, an umbrella would be more manageable.
I was thinking about making it a long-term (months long) proposal, big enough to fund about 2 conferences per month, so that we could have the funds beforehand to move fast. If unused funding would exceed a certain threshold, we d return part of it to @steem.dao.