Answer 1:
The seeds are for creating at least 128 bits of entropy, as per
BIP39 specifications...
The mnemonic must encode entropy in a multiple of 32 bits. With more entropy security is improved but the sentence length increases. We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
Just because you can "hack 128 bit" does not mean someone could come along and "steal a wallet"... anyone is free to generate all combinations of 12 word seeds right now, the wordlist is publicly available... but you don't see HD wallets being stolen left and right do you? Besides, I don't believe there is a way to reverse engineer anything in most wallets to allow someone to extract a properly encrypted seed (most wallets use AES256 to encrypt these things)... so you're pretty much covered by "256 bit" anyway
Answer 2:
You can click the "Options" button when the seed is displayed... and choose the "extend seed with custom words"... then add in 12 extra words... they don't even have to be from the wordlist... However, humans are really bad at picking random stuff... so maybe generate a 12 word seed, write it down... then don't complete that wallet... generate a new 12 word seed and add the first 12 word seed as "custom words"... et voilà a "24 word seed"