I am sorry to be slow in my response - it is the summer holiday season here and I have been having some 'go slow' time.
I am not sure how or if I can help you here. Firstly, we do NOT use the NxStage (a lactate buffered system) in ANZ and so I have little experience with it since deciding after trialling it several years ago not to add it to our home dialysis programs around the country. Apart from a single service in Far North Queensland, to my knowledge few if anyone else has embraced the system in ANZ. As a result, I cannot speak from a wealth of experience of the machine, or any of its dialysate issues.
There is a recent paper from Kidney International ... http://www.sciencedirect.com/science/article/pii/S0085253815466657 ... that points to some of the issues with lactate as a buffer in patients with liver disease ... you note that your 'liver doesn't metabolise the lactic acid when treatments are back to back' ... and clearly this may be part of the problem. You dont, however, stipulate any lactate levels ,,, though I am not sure that even if I knew them, I would be able to say with any certainty that these would prove 'symptomatic', or how.
Here, we have (1) always preferred bicarbonate as a buffer and (2) use the Fresenius 40008 S single pass dialysis system ... NB: the Baby K is far from a baby and has a large footprint ... while our S's are nice and slim with a floor space footprint smaller than the PureFlow system that the NxStage uses. Further, the Nikisso - now widely used in the Asia-Pacific - is slim and simple, as is the Belco, a system which IS in use in the US as a home system. In addition, some of the Chinese systems are small ands slinky, though as these are yet to be used here, I have no dirtect experience with their use and have simply played with them at Industry displays in China, Sth Korea and Malaysia.
When the Quanta SC+ becomes available outside the UK ... see my old blog but with a recent update to a question about it only yesterday http://homedialysis.org/news-and-research/blog/83-a-new-kid-ney-prepares-to-blast-off-the-quanta-sc ... this may prove a solution to your issues, too, as it is a single pass system, is as smaller as and lighter than the NxStage, uses standard bicarbonate dialysate and doesn't require either bagged dialysate or a bulky Pureflow-like system. It does, however, need a mini-RO, though this could be be sited somewhere nearby but not necessarily within the confined space you describe.
I am not sure that I have answered ANY of your concerns, but its the best I can provide.