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I hear you about falling back to the broadcast stations and Prime.
You really put yourself at a great disadvantage by not mounting the antenna in clear air. Any trees or buildings in the path to the transmitters will also be problematic. A rooftop mount typically gets you above the low-level obstructions, for a much better chance at reception.
Your TVFR is pretty mediocre at 20'. People who make good use of attic installations will usually have a dozen stations with 50 or better noise margin, and asphalt or wood shingle roofs with wood or plastic siding. Even then, predicting performance in an attic is uncertain.
Are there stations or a particular market that you really must have? if you insist on the attic mount, you need to go large from the start. For UHF stations, the premium choice currently is the DB8e from Antennas Direct. This antenna is panel shaped, about 4' square. You would have to assemble it in the attic if you only have limited access. You could try it, and see what you get. Not an inexpensive antenna though.
It's difficult to predict interference. Certainly an electric appliance adjacent to an antenna is not going to help. Not only will the appliance make rf noise, but any conductive structure nearby can change the tuning of the antenna, and reduce its sensitivity. Antennas work best in clear air, far away from external loading and obstructions.
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