I see a few potential problems.
48/30 is beyond the 16T capacity of the Di2 front derailleur.
also when you shift the front down you are going to need to shift the rear down 2 or 3 cogs first or you will be too low of a gear. Combining cassettes is asking for trouble. I tried it once with a. Shimano 12-25 and 11-28 for the same reason, to get rid of the 11T and gain a 16T. There was one gear that would not shift because the ramps between the cogs did not line up. I am now using a Shimano 12-30 and it works well. I have Di2 on my single and I like your idea but I would never give up my triple on the tandem. If they ever come out with a triple version of Di2 I would be interested. Agree on using steel chain ring bolts, I have had some alloy ones break.