Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision.
Scraph, that rule is compound.
First, the vessel must maintain a proper lookout at all times, using sight and sound.
Then, "as well as" refers to additional measures that should be taken, such as using radar if available.
In other words, the qualifiers you cited do not refer to the requirement to maintain a proper lookout at all times. They refer to the "as well as" measures.
I would not want any sailor to read your initial post and think that it is ever advisable to only scan the horizon every 45 minutes. I would not sail on a boat with a standing watch order to that effect.