HoMM IV Reference Guide.

Regarding the stables, the bonus isn't applied from the beginning of the week, but from the moment of visiting, and the bonus is extended by one day on the following day. If units without a bonus are added to the army on the same day, the bonus is only applied again. The bonus is only applied from one of the objects of the same type – stables, trough, oasis, etc.
External dwellings provide a 50% increase compared to city dwellings – I haven't verified this, but I can trust the source. By the way, the summoning portal also provides 1 praying mantis per week, not 2.