I don't think it's quite so subjective. At least, the edge cases you bring up would not significantly affect your metrics no matter how you handled them. MRR is a metric used for strategic planning and evaluation, it doesn't have to be exact, it's not going on your tax submissions.
You usually track MRR based on users who are currently paying for your service (summing recurring subscription amounts, as you put it). Whether their payment falls at the end of the month or is delayed a day to the next month doesn't affect MRR. Most subscriptions must be paid before the billing period begins.
Things like cancellations or chargebacks are counted separately and would not affect past MRR calculations. No you never count free trials. Yes, you count people who have a credit or balance assuming they overpaid to get that balance. If it was just given as a comp, then no.
Yes, you count people who are set to expire until they actually expire and don't renew.
You usually track MRR based on users who are currently paying for your service (summing recurring subscription amounts, as you put it). Whether their payment falls at the end of the month or is delayed a day to the next month doesn't affect MRR. Most subscriptions must be paid before the billing period begins.
Things like cancellations or chargebacks are counted separately and would not affect past MRR calculations. No you never count free trials. Yes, you count people who have a credit or balance assuming they overpaid to get that balance. If it was just given as a comp, then no.
Yes, you count people who are set to expire until they actually expire and don't renew.