My titled question was, can a planet without one or more moons be
Habitable? Totally. Would life evolved in it? Probably. Always assuming that other conditions adjust to it…
In more depth, how would that planet be affected overall, without the
moon to affect the tides
Just life marine without tides XD.
and without providing light during night?
Light is useful, but it is not necessary for life. It's highly probable that the first life on Earth emerged without photosynthesis by using chemical and thermal energy.
How badly would oceans/seas be affected? could life be sustainable
within them (fish, algae, etc.)?
Not badly, probably would be different. Also, there are other phenomena that could make water currents (winds, thermal, temperature gradient, etc.).
Lastly, would the atmosphere be majorly affected, much stronger or weaker >than would otherwise be possible on said planet?
The Moon does have an affect on the atmosphere, but it is not vital for life. The magnetic field has a more important effect.
I already know that the heat reflected by the moon is to insignificant
to truly affect the heating of the planet.
Yup. Probably it has a more important effect in protecting from meteoroids.
From a writer's perspective, a planet like this is an interesting
concept because to humans it would be a foreign idea; stars would be
that much more visible at night as well as it would be very, very dark
It would affect, not just aesthetically, but also the culture of the inhabitants. Just think:
- How many poems and literature refers to the Moon.
- The Moon it is a small step into space exploration and astronomy. Without it, probably those things would be more slow.