1) Yes, of course. You can always add/remove guests as long as you remain within the room occupancy.
2) if you're looking for a very specific room type that is very limited, then you might want to do that, but it's certainly not necessary to book a year out.
2b) The question of whether you'll be able to add a discount later can be problematic. You may be able to, but might not. Discounts are specific to resorts, room types, dates, and inventory. Generally, you can only add a discount if that discount would be available for the exact reservation you already have. If you couldn't book a duplicate reservation with the discount, you won't be able to apply the discount to your existing reservation. In that case, there may be a different resort or room type that does have an applicable discount and you could change to that option.
Hope this helps
Steve