At least with Mobile Suica, it is tied to your iCloud account, so you do not really lose the balance when you remove the card from your iPhone/Apple Watch or if you change your iCloud account. You can add it back even after you remove it from the device before and the balance will be there once you log back into the iCloud account that has the Mobile Suica. I am guessing Mobile Pasmo works the same way.

I just went into my Apple Watch settings for Wallet and try to remove Mobile Suica on my Apple Watch and the screen even said with subject "Value Remaining on Card" and description "If you remove this card now, you can restore the value later in Wallet" so the assumption the left over balance is lost is simply not true at all.

So I think you are safe either way. When you log out of your iCloud account, I would think the Wallet cards / etc would be removed.

For pyhsical cards, you can buy one of the registered ones with your information on it which would allow you to recover the balances if you lose the physical cards, but if you don't have a Japanese address and identification to prove it is your card I am not sure how much advantage that would be and I believe you would also have to pay some small fee (a few hundred yens) to be re-issued another Suica/Pasmo card.

Topping off Suica/Pasmo will be in increments of 100 yen, just tested with my Apple Watch. You don't have an option to top up the exact amount.

A lot of stores / restaurants do accept Suica/Pasmo, in addition to the convinence stores / vending machines, but I don't know if "most" is accurate. Small shops probably still only accept cash.

It is instantly loaded and you can use the amount you just loaded immediately.