So, first of all, the sentence （私は）月曜日にいつも兄と一緒に食べます。 is grammatically totally fine and would be comprehended in the context you give. This would probably best structurally translate as
"I usually/always eat with my brother on Mondays"
However, partly because Japanese is a pronoun-drop language, and partly because of the situation -- where the speaker ...
It would be best to denote the house as both the location (of the rooms) and the topic of the sentence by using both.
In my house [←topic], there are three rooms.'
Colloquially: 'There are three rooms in my house.'
Both こんばんは and こんばんわ sound the same as the ha (は) in こんばんは sounds like wa with its current context so basically there is literally no difference between the feeling that わ adds since it doesn't make the it sound any different, just making it clear not delivering hate.