Hotel decisions

In my experience:
Yes to question 1 because it is a specific type of room usually at a higher cost than a no-window room.
Probably to question 2, but I often have the reply that it will depend upon availability upon arrival.

In European accommodations what you’ll often find is that two twin beds described are two separate mattresses on a single bedframe. Push the two mattresses snugly together and the two twin beds essentially become one full bed close to the size of a queen mattress. The mattresses are so close together that it’s difficult to tell it’s not a single mattress.

Sometimes you run into two entirely separate twin beds across the room from one another, but that doesn’t happen as often as the two mattresses on a single frame.
So, I wouldn’t be too concerned about booking a room with specific mattress types because most rooms have beds that can be either. (If the issue is a snoring partner— that’s another story)!
With regard to windowless rooms, those sound like they may be basement rooms or attic rooms. More rarely would you find a windowless room on a floor with windowed rooms. I understand your concern about that because I would find a windowless room unacceptable.
Look at the number of rooms the property has. If the property is small and only has a handful of rooms and you think you might be arriving late in the day to check-in— I would reconsider reserving at such a place. The reason being is that the last guests to arrive late at night might be told “ We’re sorry but this is the only room we have available tonight.”
You’re kind of stuck in a situation like that.
If you’re reserving through a site such as Booking.com or Hotels.com, look at the rating number ( ie 5.5 or 8.5 for example) and take time to read the guest reviews of the property. They will give you a pretty good idea of how concerned the proprietors are about keeping their guests satisfied with their stay.

...some hotels,you can make reservations, where it says 1 queen bed or 2 twin.
There too,if we check 2 twin,can we be certain that's what we will get.

It's been my experience that in a number of cases, a queen is just two twins pushed together and made up as a queen.

In those situations, I send the hotel a message stating my concerns and wishes.
If using booking dot com, this works really well as well as direct emails to the hotel.

You have received good answers to your questions. In my opinion, I would never stay in a hotel that sold windowless rooms. These must be one or two star rated hotels.

No, they’re not all one and two star rated hotels. I just looked at a Marriott hotel in London that just offers room without a window.

I stayed in a very well rated (and recommended here on what was then the Helpline) hotel in Bologna which room had only a skylight and no windows. Hated it. Good brekkie though. Won't return there.

Windowless rooms exist in many mid-range hotels in Scandinavia too, though I would never book one.

I’ve seen the windowless room option on some very nice places to stay, such as a converted palazzo to a B&B. I do reserve a lot of my reservations through Booking.com, and they can be seen in the options for reserving one specific room - not just a type of room. Also, sometimes a window is just an interior window, not a window looking at a site or out the side or back of a building. I’ve specifically chosen those when I am going someplace very noisy, such as a festival but still want a good night’s sleep.

When I have someone sharing my hotel room, I try to remember to send a reminder about needing two beds a few days before arrival. That can't hurt. I also mention it during check in. Just last summer I ran into a hotel that had ignored the two-bed request made at the time of reservation. Fortunately, the room we had been assigned had bedding that could be set up either way. It was annoying that we had to wait quite a while in the lobby for the bed to be split apart, but much worse things have happened to travelers.

If you arrive late in the day, there is a risk that all you'll get is a room sleeping the correct number of people, not a room with all the features you were expecting. While staying in a low-end hotel in the Balkans that had some air-conditioned rooms and some non-air-conditioned rooms, I was assigned to one of the latter despite having reserved one of the former (at substantial extra cost). That was easily resolved at the counter.

On very rare occasions a hotel (not just in Europe) might have no room at all for a late arrival, because some people unexpectedly didn't check out. However, I travel a great deal in Europe (probably over 250 hotel stays since 2015 alone), and I've never failed to find a room available at check in.

Gone are the days of asking to see the proposed room before accepting it.

That is not true. We have always found that we could check the room prior to paying. Just ask.

When I travel with my son I book us one "twin" room. Once in Bratislava we found that the room had one double bed. We just went back to the front desk. At first, it seemed like it was going to be a problem, but when I could show a copy of my booking email and it specifically said 2 twin beds, we were upgraded to a very large and luxurious room with 2 twin beds.

Another instance in Luxembourg, the twin room was two twin bed mattresses in one bed frame. Otherwise, we have always been given a room with 2 twin beds. Frequently the beds are arranged right next to each other. We don't hesitate to move the side tables from the outside to the inside of the beds to give ourselves more space.

When booking, I look carefully at the pictures of the rooms on the website and on Tripadvisor.

I realize that different countries have vastly different building codes, but for safety I would never accept a hotel room with only the one door for an exit.
Even a skylight could be used as an exit in a dire emergency.