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You can optimize the peak gain of the antenna toward lower frequencies by making the elements longer. 16.7" would favor channel 7 instead of channel 12.
A perfectly tuned dipole has a theoretical gain of 0 dBd if everything was perfectly efficient. Baluns are never 100% efficient, and there are bound to be slight impedance mismatches, so your net effective gain is probably more like -2 dBd.
The multi-element Yagi antennas like the YA-6713 and Y5713 are able to achieve about +6 to +7 dBd, so I do expect them to perform better than a simple dipole. They also have better directionality, so they might cut down on some multipath interference if that is one of the factors causing you to lose channel 7.
To combine the VHF and UHF antennas, you should use a Pico Macom UVSJ or equivalent. This should limit your combiner losses to only about 0.5 dB, and it will prevent mutual interference between the two antennas.
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