is doublelift coming back during summer split

Yes and no. While he did "say" he was coming back for summer, many people (aphromoo included) are under the assumption that he might enjoy streaming so much (due to it being more profitable/less tedious when it comes to hours) that he could just permanently do that.

yes

It's up in the air. If TSM is having a hard time during the Spring split he will take Turtle's place during a span of games until the team seems fit to let Turtle be the starter again. Similar to SKT's Blank and Bengi.

The Summer Split with D-Lift returning is up in the air, but I personally think he will return because of 2 reasons, A. I don't think Spring Split TSM will be that great and ranking them 3rd best record overall which in turn will cause D-Lift to possibly comeback sooner than later and it's easier to have a recent ADC perform with the same team.
B. It's the last split before Worlds and who doesn't want to play and get good build synergy etc with a team possibly having a chance to make Worlds and try to win the whole thing.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^Question - Opinionated answer

I hope D-Lift does comeback because it would be a final chapter in his book mainly because he's been playing for about 6-7 years now and once a player says " I"m just going to take a small break" their really saying " I'm retiring soon" and since I'v been watching D-Lift play since late 2011 it would be right to most likely end my own experience watching the LCS with D-Lift possibly retiring. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Foolish backstory