PS: If selling SBD can cause harm to Steem's price, why doesn't Steemit Inc. keep SBD limited to Steemit wallets and list only Steem on exchanges? (provided SBD stays convertable to Steem).

this cannot be done, both tokens are freely exchangeable.

What will happen if I do not maintain 1:19 SBD to Steem ratio?

for you it shouldn't have any impact

What if my SBD balance is negligible? How will it affect me?

you will just have less SBD. The balance between SBD and STEEM is important at a higher level.

When we talk about SBD:Steem ratio, is SP considered Steem? I think yes but questioning is good.

I think it's only about liquid STEEM. SP cannot be traded unless powered down.