What exactly is it that makes the editor so hard to use in that regard?
It's not "hard" so much as it's just easier for me to do it the other way. I can copy paste whole chunks of buildings and then just shuffle a few rotations and viola! I suddenly have a new wall for my structure. In contrast, the editor still does not have any sort of copy/paste feature... you can't duplicate props. The exposed tab for manipulating vectors is still buggy, doesn't always exactly line up props to the value you type in and doesn't place the cursor where you indicate, instead always placing it at the start of a line. While none of these things are terrible, its still a fantastic tool, it is just ultimately faster to do most of the structural work in the objpos.
And 2nd question is: manually editing the objpos can result in perfectly aligned blocks that do'nt require arches and other connectors to mask the edges?
So long as you find the correct value for any prop, then afaik yes.