Chains don’t actually stretch, they do wear over time, but after the initial settling of a brand new chain, they’ll not make a significant difference.
Half link chains are all toss ime, fine for BMX’s when you’re grinding, but other than that they are a big old pile of ****. Just use a decent 8spd chain and all will be well.
EBB’s do need to be tight and if you can set it up so the BB is in between the junction of seat tube and down tube, then your weight will naturally try to make the chain tighter if the eccentric moves.
I lurk on here most days, but rarely post as everything has already been said most of the time 🙂
Tim.