Should the borders of non physical landuses of like retail and commercial be connected to the street?
Thanks
No
You should not connect landuses of anykind to ways, try to make them independent from everything else. They are what they are imaginary borders. With one exception the landuse of a specific farmer who uses his land for several things, such as farmland or meadows.
No.
Gluing is good when there is border between features, and moving/tweaking one feature should also do the same with all glued ones
Say border between landuse=residential and landuse=industrial, with no overlap and nothing between them. In such case gluing is helpful as it makes future edits tweaking geometry easier.
Or two joined buildings.
gluing is bad when border is somewhere else (so gluing to road centerline is a poor idea), something is between this areas, or moving one should not move other areas (typically true with administrative borders).
Other example: if there is lake and there is forest, with forest not reaching lake (because there is a beach) - then do not glue them.
On a similar topic. If there is houses (or shops) across the street, do you make two landuses for each one or one that covers both?
Yea that case is clear cut.
Depends on the level of detail you are striving for, and the state of the surrounding area (which you may wish to match).
For landuse=retail another consideration is if the street itself is part of the shopping area (which is often the case with highway=pedestrian), or if it is a normal road that separates two sides.
The wiki gives some explanation about the level of detail: https://wiki.openstreetmap.org/wiki/Land_use