Can you do a seine survey and post some pictures showing the body condition of the BG and/or LMB that are present in your pond? A rod and reel survey with pictures showing the body condition of any larger fish you catch would be helpful too. This should give us some clues to the current fish population dynamics going on.

I wouldn't be worried about the bullfrog tadpoles, around here bass won't eat them if there is an abundance of other better things to eat, BF tads don't taste very good to LMB. Having a lot of BF tadpoles doesn't necessarily indicate that your LMB are all gone, it just indicates that you had a good tadpole hatch awhile back. My guess is that the alder saplings that you added provided sufficient cover for a few adult bullfrogs to avoid predation by your bass which then resulted in a good tadpole hatch, this is likely the reason you currently have an abundance of BF tadpoles. Ponds with good frog populations usually have decent amounts of aquatic vegetation present that provide cover from predation.