Thanks for the inputs @ishq74
1. Yes it will remain valid but unlikely they will open again within 45 days. (in this case 31 days assuming applicant registered same day)
2. Same here, they have only 6600 slots for whole year but for Express entry it is only 2500 (2017 stat) and apart from hcps they have other programs too. It is highly unlikely your NOI will remain valid until they open the program again.
Don't waste an opportunity if you get one. Be ready now with all documents. Read document checklist in application guide and get everything ready.
Hope it helps, Good Luck!