It is correct to say “d’la mer”, but “de la mer” is just as correct. It might sound a bit strange though if you do not do that consistently but I am not sure I would pay attention to that myself for just an occasional lapse;

• Je’m souviens d’la mer, pas de la montagne.

You must, however, know that, when you use these shortened pronunciations you put yourself in a certain category of speakers, the speakers from Paris and some of the northern regions; so, although nowadays people in the South, that didn’t use to have these speech habits and looked down upon them, are not adverse to this way of speaking and sometimes even use it, will identify your speech as that of the people from Paris and the North.

• i’t’l’a donné ?

You do have to release the plosives (t, d); there is no release stage for l as it is a liquid; you can of course do away with the t or d and then you are talking very fast, too fast for your speech to be called normal or standard; so, no, you can’t gobble the t’s and d’s, they have to be exploded, even if only so slightly.