The "A" in Roth IRA stands for "arrangement", not "account".If someone does do a tutorial on Roth conversions, it would be helpful to include a chapter on distribution considerations. Specifically, if a younger person is doing conversions and may want to take some distributions prior to age 59.5, they should keep careful documentation on how much was converted and when so they can avoid having a penalty caused by dipping into earnings as opposed to conversions. I recently saw advise to create a brand new Roth account each year so that this is crystal clear.
The five-year period starts on the first day of the tax year for which you made a contribution to any Roth IRA account, not necessarily the one you're withdrawing from.
This was settled a few years ago when the Supreme Court & IRS ruled that you have only ONE IRA (and one Roth IRA) no matter how many different accounts it is spread amount.
Figuring the earnings is simple. Current value of ALL Roth accounts minus the total amount you have contributed to the Roths.
Same as for RMDs from taxable IRA accounts. You don't have to take an RMD from each account, you just have to take the total RMD from whichever account(s) you like.