Can a beginner cyclist commute ~40km (round trip) to school/work by bike? I know this kind of question has been asked several times here (I've read those already), but I'm still uncertain and I have a few other questions.

I hardly ride bicycles. I've gone night cycling for 5+ hours before (not sure what distance that translates to) and I don't remember it being especially difficult except that it was pretty uncomfortable for my butt, and I've read that people actually do travel 20km to work daily, so distance itself shouldn't be a problem right? And feasibly how long would it take to bike that distance for the commuter-level people?

However,


I live in Singapore, so it's extremely hot and humid. I'm not sure if this is pertinent at all but I'm wondering if it makes cycling long distances unbearable
Although we have bicycle roads scattered around the country, it isn't a cyclist-friendly place. I'm aware drivers here detest it when cyclists cycle on the road, and to add to that I'm not confident about cycling on the road either. I'd much rather cycle on the pedestrian path but I heard it's rude to, or is it okay as long as you maintain a respectful speed and not get in the way of pedestrians?
Since each commute is around 20km, if I'm not wrong a road bike would be better, but I'm more inclined towards city/cruiser bikes because of their appearance. This may seem like a bad reason but if it's only going to be a bit more uncomfortable then I'm willing to suffer that. Will Creme or Linus bicycles suffice? Like Creme's Molly series. If they're not good enough, are there any decent road bikes that look like that?
Bike theft here is common enough for it to be a concern for me. Since I'm probably going to be leaving my bike unattended for several hours (may be up to half a day), if I use hardened steel bike locks and U-locks will my bike be safe? I'll be parking it in my university but I'm not sure if that is going to deter bike thieves at all. From what I've read online bicycles are stolen so much it almost sounds inevitable, and I really don't want to experience that. Also, handcuffs are illegal in my country so the Master Lock street cuffs probably are too- are there single cuffs for the bike wheels and are they reliable? They seem pretty sturdy to me.


Just in case anyone wants to know, I'm considering a bike because travelling the same 20km by public transport takes 1.5h and requires me changing buses/trains several times, which is too much hassle.

Thanks!
I have been riding to work for about 20 years now and my round trip distances have ranged from 35-75 km (50 km currently). It will be difficult to jump into riding 40 km every day, 5 days a week if you have little riding experience.  Fatigue can quickly accumulate depending on how hard you ride each day.  Instead, I would suggest starting with just a few days a week and work you way up.  You will also need to plan for recovery weeks in order to keep feeling your best.

In terms of your specific questions, here is my take


  
  I live in Singapore, so it's extremely hot and humid. I'm not sure if
  this is pertinent at all but I'm wondering if it makes cycling long
  distances unbearable
  


Depends on a number of factors such as


The time(s) you are commuting to work;
How well you stay hydrated during riding;
Your tolerance for humidity and heat (which in turn is affected by a large number factors such as your fitness, hydration, sleep, genetics, clothing...);
How fast you ride / and how often you take breaks; and
The difficultly of the route you ride (e.g., flat vs. hilly).



  
  I'd much rather cycle on the pedestrian path but I heard it's rude to, or is it okay as long as you maintain a respectful speed and not
  get in the way of pedestrians?
  


This is really specific to the region you live in.  By-laws may exist that either allow or prevent this, as well as social convention may or may not allow this. The specific laws governing cycling in Singapore didn't make explicit mention, but this blog post seems to suggest that it is actually encouraged.  That said, a comment within that article suggested otherwise.  Perhaps it would be worth asking this as a single question on stack exchange.

Some other interesting tidbits from the laws governing cycling in Singapore 


Your load cannot exceed 18 kg.  


(Good thing most textbooks are digital now, I would have been in trouble during my undergrad.)

You must use a bike path if one exists.



  
  Since each commute is around 20km, if I'm not wrong a road bike would be better, but I'm more inclined towards city/cruiser bikes
  because of their appearance. This may seem like a bad reason but if
  it's only going to be a bit more uncomfortable then I'm willing to
  suffer that. Will Creme or Linus bicycles suffice? Like Creme's Molly
  series. If they're not good enough, are there any decent road bikes
  that look like that?
  


Because you will be spending a lot of time riding, getting a comfortable fit should be your top priority.  Fit is also a very difficult subject in cycling as there is no such thing as the perfect fit, only a balance of compromises. I personally find being too upright uncomfortable (it places all your load on your sit bones), but also feel that many road bikes are too low.  20 years in I am still working to improve my fit.  Before you write off all road bikes, look at the "endurance" type road/touring bikes.  These will have a taller frame stack (vertical distance from the bottom bracket to the top of the head tube) than the "race" road bikes and will be much more comfortable for most rides while still rolling efficiently. 


  
  Bike theft here is common enough for it to be a concern for me. Since I'm probably going to be leaving my bike unattended for several
  hours (may be up to half a day), if I use hardened steel bike locks
  and U-locks will my bike be safe? I'll be parking it in my university
  but I'm not sure if that is going to deter bike thieves at all. From
  what I've read online bicycles are stolen so much it almost sounds
  inevitable, and I really don't want to experience that. Also,
  handcuffs are illegal in my country so the Master Lock street cuffs
  probably are too- are there single cuffs for the bike wheels and are
  they reliable? They seem pretty sturdy to me.
  


No lock is theft proof. With the right tools any lock can be defeated. That said, some locks are a much better deterrent than others, plus you can mitigate your risk by carefully selecting where you lock your bike, as well as the general appearance of your bike (less flashy, means less of a target).  I remember people at university spray painting their bikes all sorts of horrible colours to deter theft.  I personally just found some places that had high visibility with lots of people. Of course, like any advice your mileage may vary.