Yes, if you advance to a higher league you will get end of season loot chests for that league. The end of season loot chests are based on your highest league achieved in the season. So you could move to silver 3 on the last day for sure, and then your end of season chests will be for silver.

Your daily focus will still be a bronze one then if you move to silver in the final day, unless you get a new daily focus after you already advanced to silver and before the season ending reset put you back in bronze. But even if you do get a silver daily focus, that might not be so bad to not get many chests for it if your end of season chests are all silver.

Well I'm not sure if you need the level 3 summoners and monsters at the last day of the season for silver 3, but it's just a guess as I only play in silver 3 at the beginning of the season when I'm climbing back up. I think you could move to silver 3 at the last day of this season and then see how it goes before deciding if you'd already level them up. If all your cards are maxed for bronze league then I could suggest working on getting some to level 3 though for sure!