I’d recommend searching about their fees this because it’s going to vary a bit based on your local currency. Their documentation on the topic is easy to read and answers your first two questions better than I could put it.
I’ve rarely had to interact with support so I couldn’t give a useful response about that in earnest. They do have local support in the two major countries in which I’ve interacted with them and it’s been fine.
The KYC process is standard for a digital money account AFAIK. I signed up in 2017 originally to handle a one-off transfer between local bank accounts in different countries, so I’d not have bothered investing much time in it if it was a hassle. I haven’t had to re-identify myself or think about it since, despite migrating across several countries, starting to use the physical card etc. I imagine I gave them my government ID though.
Yes, for a loading method which takes time to complete the conversion rate will generally be locked in from the time the transfer was initiated. You can always preview the received converted amount. Although the transfer method may vary depending on availability in the recipient account country, the various timeframes and fees (where applicable) have always been previewed accurately.
For myself I have set up MFA for payments via the app, so I will routinely be required to use the app to authenticate a payment. However other MFA methods are available. I can’t think of any other function the website itself doesn’t do.
Given there is no additional fee for converting at a foreign point of sale, I just load up in my home currency as there are free/instant methods available and convert to whichever destination currency at point of sale, ensuring to select not to have the balances converted by visa/master at terminals which have that function.
Hope you enjoy your time in Japan!