I'd say that in Czech the accent is pretty much always on the first syllable. Some people even say it is essentially on all syllables.
But it definitely won't be mainly on the second or any other than the first, unless it is a foreign word (and even then most Czechs will probably pronounce it as though it were a Czech word, i.e. with an accent on the first syllable). Czech is very simple in this regard.
I'm pretty sure they mean accent as emphasis here, not accent as in "accent mark". Now, if there is an accent mark on the letter a -> "á" then yes, it would make the letter longer.
No. Not in this case. Žena only really means wife when you add "my" in front of it. Sounds kind of sexist but it does not feel that way in Czech not to mention "My man" a.k.a. "můj muž" also means a husband.
But even adding "moje" here would not sound right. "Moje stará žena" would be puzzling for Czechs. Are you being politically incorrect and calling your wife old or using weird word for ex-wife. Ex-wife would be "bývalá manželka/žena"
Yes. Interestingly, in Spanish only "mi mujer", or "my woman" is correct, whereas "mi hombre" is never used (mi esposo, or my husband is used instead), which makes Spanish kind of sexist :-)
Also 'mi marido', literally 'my married'. Note, never 'mi marrida' because its not a formulation used by men to refer to their wives, only the other way around
Same thing in French with "femme".
Ma femme = my wife. Une femme = a woman.
Is it the sme with other words like boy, man or girl?
'Ex-wife' = exmanželka or bývalá žena or bývalá manželka or if you want to sound super casual, you can use the very common, very informal bejvalka, just keep in mind, it's not standard Czech.