Next Due Date on Inspection Tag

Yes, this is the best way. If you put only the date of the most recent inspection, and the owner/operator is unaware of the correct interval, he/she won't know when the next inspection is due. Stating it directly prevents confusion.

For first question- already answered - all ours are with due date. on the equipment. The certificates (filed away for reference) have both the dates.

2. It all depends on the usage for us it is annual.