To do both of these on an iPhone . . . Hit and hold your ?. You will see there is the option to do a ? and an ¿. Select one.
To add a ´ hit the letter you want accented. Hold down and look at all the choices they give you such as the O in versión. You will see several accents to choose from.
But, actually, my question had been rhetorical, and I'm sorry to have bothered you.
1. I don't own a cell phone and don't really understand what a "tweet" is.
2. I didn't actually think that it was even possible to insert special symbols (Greek letters, mathematical symbols, letters with umlauts) into "tweets." I thought that, instead, everyone just used "u" for "you," "r" for "are," etc.
Regards,