I do see your point about the pipework, but bear in mind, if you have a couple of lengths of 22 splitting off to a few rads, and the radiators are off, there will be no flow to heat up that section of pipework. On a two-pipe system, it isn't a loop as such.
Of course, a manifold system does have less water in it, but you may find that the heat loss from several lengths of narrow pipe is the same as that from one piece of 22. Unless you really only intend to heat one room and the house is very large? Of course, it is up to you as only you know the details and what is important to one person is not important to another, so you will ultimately choose what you feel is right.
That said, it sounds like you are giving this some thought, and I look forward to hearing of your progress.
Of course, a manifold system does have less water in it, but you may find that the heat loss from several lengths of narrow pipe is the same as that from one piece of 22. Unless you really only intend to heat one room and the house is very large? Of course, it is up to you as only you know the details and what is important to one person is not important to another, so you will ultimately choose what you feel is right.
That said, it sounds like you are giving this some thought, and I look forward to hearing of your progress.