Zeno, In answer to some of your questions -

For multi-band working, ladder-line or preferably open wire line is almost
essential.

Overall antenna length is very non-critical. When using a tuner you can be
many feet adrift from the theoretical calculation without any degredation in
performance. A change in length affects only the radiation pattern which
for practical purposes can be considered omni-directional anyway.

For multiband operation be prepared to make relatively small changes in
length of feedline as part of tuner operation.

Use a simple choke balun at the transmitter end with a balanced feedline.
Make no attempt with a voltage-ratio balun to match feeder input impedance
to 50 ohms. Leave that entirely to the tuner.

A choke balun for 160 meters consists of a 1.5" to 2" diameter ferrite ring
wound with 8 to 12 turns of 18-gauge flexible speaker cable, or similar.

A coax feedline means lossy operation on other than the fundamental
frequency.

A balanced feedline near to phone or power cables will cause no trouble
unless nearly touching over a long distance.
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Reg, G4FGQ