But the source signal tapes are missing? (before conversion to NTSC)
No such conversion was needed for the Apollo 12-17 TV transmissions.
Was there any footage on the tapes that was lost, in a format that we no longer have?
No.
And how do you know there wasn't footage that simply wasn't converted to NTSC? (meaning that the source formatted tapes have footage no one has seen)
Because no such conversion was needed, and because there is continuous audio and transcription for all missions, and the TV transmissions were called out in there and match up to the video material. Why do you feel the need to invent this possible missing footage? The reports about the Apollo 11 tapes are very clear about what they contained that was of interest. I fail to see where your basis is for concluding thre might be additional TV footage we never saw.