My reading of the issue with conductive masts in the centre of dipoles is that while it is undesirable from a purist’s viewpoint, it doesn’t really cause a big problem. The fields from the antenna, if symmetrical about the mast, would be equal and opposite in polarity.
My “temporary” home mast is a 6m steel pole. I have an outrigger offsetting the feedpoint for my 4 band fan dipole by about 40cm. The coax feed plugs into a balun and the antennas are all attached to the balun. A halyard running through a stainless steel sheave (and its nylon pulley) is attached to the outrigger and allows for antenna maintenance and mods without lowering the mast. The main mechanical drawback to this arrangement is that the coax feedline is hanging from the feedpoint (the balun actually) and cannot be strapped to the mast once the mast is up. The weight of the coax is partly supported by a rope harness attached to the feedpoint insulator, to take some strain off the coax connector, a PL259 in this case (one of the few I use).
Re 160m I read comments from the top band specialists saying that low dipoles (indeed any horizontal wire antenna including the loops) are very inefficient due to the high angle of radiation and the low inherent radiation resistance, which is largely ground loss resistance. They say that a loaded vertical is better for transmitting than any low dipole. I have not tried that myself, but I think it is a very credible claim.
We can read on this reflector of efforts to get an 80m dipole extended for 160m, the low signal reports received seem to support the claims about low dipoles.
Support for the effectiveness of loaded verticals comes from some VKs I know who have tried them, including Tony VK3CAT, Dale VK1DSH, Dimitris VK1SV/VK2COW, Peter VK3YE and others.
So while not saying the low dipole won’t work, I suggest if you want better performance other antennas not so affected by proximity to earth may be a better option. Perhaps not achievable in your initial setup to get a few bands working remotely.
Thanks for raising this subject, it is of interest to many city dwellers.
73 Andrew VK1DA/VK2UH