You only say when you first and last entered on your METV but not how long you stayed each time you entered and whether you tool the 30 day extensions at all. These are the things that can get an entry denial not the number of times you have entered.
No, that is the period when the Visa must be used not the duration of the stamp that you get on entry. Both a Non-O (90 days on entry) and a Tourist Visa (60 days on entry) are valid for use for 90 days from the date of issue.
If you are leaving for Singapore before your next 90 day report is due then you will not be using Immigration so you can leave the TM30 until after you return