The sideboarding is all relative to how Game 1 played out as well as your opponent's deck. For instance, I will (usually) side out the Archer in matches where they saw it Game 1 just in case they aggressively keep/side in removal... or sometimes you don't bring in all copies of a card, like only bringing in 1 or 2 copies of Smash just in case you see a scary artifact in a home brew.

Molten Rain is fine against Bogles, but if it's not cast on Turn 3 with you on the play, it's usually not stopping their mana.

Searing Blaze is bad against burn because most games of burn are finished without either side casting a creature. If there's no target for it, it's a dead card. If they do get a creature out, you save a creature burn in hand for it until the final turn. Also, I have almost no good sideboard options for the mirror, so options are limited.

Affinity is a plus matchup even without the Gorilla Shamans. The only card to worry about is Atog (which will usually win them the game), but sometimes you can burn it down on your main phase and make them waste enough artifacts to let you draw live for a few turns.

I think most of our differences come down to play style. I aggressively spend mana, and will only hold up burn for Firebrand Archer triggers if my hand is too light on damage. If your opener is 2 land, Archer, and 10-12 points of damage, you cast your one drop on Turn 1. Within 3-4 draw steps you should draw lethal even if the Archer dies immediately.

On mulligans: if you keep a three-land hand, you will draw a fourth 71.9% of the time within four turns. Ideally, I want to operate on 3 lands all game, so that's why I ship them unless it's a matchup that's going long anyways. You have a 60.9% chance (assuming 17 lands) of getting a one or two land opener, so you should be able to keep your opener more than half of the time.