Starter decks are relatively inexpensive considering their contents when they can be found at MSRP, making them a much easier way to start playing than building a deck purely out of booster packs or aftermarket singles
Cold-Eyed Selkie, Herald of Secret Streams, Iridescent Hornbeetle, Managorger Hydra, Sakura-Tribe Elder and Trygon Predator have all been subbed in
Yes, you can add a companion card to your partner with commanders as long as you meet the deck requirements for companion
Of course theres the drawback of them entering tapped, but they were tapped anyway from paying for the spell
Lilysplash Mentor's ability costs three mana, and can only be done at sorcery speed
Playing a land also won't cause prowess to trigger.