45% silver is fine for fillets, but it doesn't normally build a big fillet, so you have to get a pretty good one. It doesn't get used much because there are structural pitfalls relative to cooling, cracking etc...and at the end of the day it can end up looking even more naked than TIG, which isn't the current style.
I don't know anything about the science of the the thing, but what seems to happen with silver fillets is that the colder the flame the hotter you have to heat the metal and the dirtier the result. When you have enough heat, the metal doesn't need to be heated as much in one place to get the process to flow, and when it flows, it pretty much jumps into the joint in a nice even fillet without the need to really build it much.
I find Mapp a little sketchy on small tubes, let alone frame sized tubes. What people end up doing who pushing a little torch too far is they end up with an individual area of deposit they could heat enough, and then they build another. What results is rather like a series of linked spot welds. The difficulty is that areas outside of a "weld" area tend to accumulate contamination. It is extremely difficult to remove this and keep going and indeed there is probably contamination one can't find in recesses of the joint. People have made long lived bikes this way, but it is a little risky.
I assume you have the correct flux for silver? The one in evidence isn't the one I use, but that doesn't mean anything.
Anyway, congratulations on setting out on your quest. It is fun to do, and worth the (craigslist?) investment in a hot enough torch.