Only listings that are created during the promotion are free - if you create 5000 listings one month and get the promotion again the next month, only 5000 new listings will be free and the 5000 from the previous month will incur insertion fees (unlike the basic free listings you get with a store package).

Generally, all additional features incur the same fees, although the current promotion includes half price subtitles ($1 instead of $2), which is valid for the initial duration and will be charged at full price when it ticks over if not removed before the 30 days is up. 

TBH I don't think this particular promotion is all that beneficial to many store holders. The first one I received included auctions, but none of the other ones have. 

I'm pretty sure not all store holders get the promo,  and I know when I got the one that included auctions, some other sellers just got BINs. It would be pointless for anchored store holders to get 5000 free BIN listings as they're already free with that package (@$499.95 subscription fee p/m, but with reduced FVF). 

If you have 5000 items, personally I think you would be better off basing the decision to open a store or not based on all the known applicable fees rather than a possible (seemingly random) reduction / promotion. Try the fee illustrator to see if a store package would benefit you: http://www.fees.ebay.com.au/feeweb/feeillustrator