It's very hot (40 Celsius at least to be an onsen I think). We also had to take cold showers in between to last. Even your face sweats. I wouldn't do it pregnant.

If you book a private onsen no one sees you and no one will know about the tattoo. I just did it and had no problem. Sure at the check-in they ask if you have got one but you can just say "no".

It´s in their website and they do answer emails in English. We made the reservation through email all in English. I´m putting spaces in the email so they´ll let me post this (in the first space you should write at and in the second one dot): yuryo hakoneyuryo jp

No non c’è bisogno di prenotare. È una struttura molto grande e molto organizzata . È bellissimo

If you are doing it for overseas, it may be difficult to do it online, when I went, I rocked up really early, (opening time) and managed to book whichever private bath I wanted, and a massage. But booking a room maybe be more challenging if you don’t plan ahead.

We arrived early morning (opening time) And got a private bath straight away, although... I’ve heard if you arrive midday or later in the day, you won’t find one available. The less expensive rooms will probably be unavailable, but the more expensive rooms (which we hired) may be more readily available. We attend on a Saturday. It was also raining. I guess weather could be a factor... be safe, if you really want to get a room go early or book in advance.

I didn’t make a reservation, no difficulty with seating, didn’t have to wait. There is a fixed menu offer for lunch that was very good

Not really sure about the last train timing but you should be able to find the information via Google.

Yes there is a changing room. The private onsen consists of 3 areas: the living room, the changing room, and the pool (onsen) area.