If the date of issue for my MeTV (multi entry tourist visa) is, say, 20 September, does that make the latest date I can use it to re-enter Thailand around the 19 March and I could stay in Thailand for another 60 days, so needing to leave Thailand  around 17 May, without the need to visit immigration?

 

And if I wanted to extend it by visiting immigration I could do that, giving me up to around 17 June?

 

 Thanks for any help. 

Your visa will show the last day you can use your visa. That would be 6 months for the day it is issued which is what you show in your post.

You could get another 60 day entry from your visa on the day it expires and extend that for 30 days.

Thanks for your reply. 
 

So are the dates I mention and the assumptions I make more or less correct. 

9 minutes ago, sureno said:

So are the dates I mention and the assumptions I make more or less correct.

Yes