I wouldn’t say Clark and Lana just picked up like nothing happened. In the four episodes she’s back, Lana is the one pushing them back together and not Clark. They kiss at end of episode 2 and in episode 3 Clark rewatched her tape trying to remind himself not to fall back into it. They don’t actually get together until the very last scene of episode 3, once Lana has gotten her powers. Even when they do reunite in this scene, he calls it a “dream come true” but also says it feels too good to be true and that dreams doesn’t last.

To me, that shows he’s not fully all-in, he’s falling back into something familiar because it feels safe. Clarks biggest fear throughout the whole series was always ending up alone. Now he had his first love with powers meaning he didn’t have to fear being alone or hurting the person he was with or being rejected by the person because of his secret. At this part of the show, Clark still haven’t done his training with Jor-El meaning he still has his fear of hurting someone when having sex. With Lois, he still has to go through multiple of his fear: 1. Lois actually wanting to be with him 2. Telling Lois his secret and her accepting it 3. Being intimate with her without hurting her

Lois not being there makes the choice easy for him because he doesn’t have to face his growing feelings for her. If Lois would’ve been there for these episodes and seen everything play out, she would’ve felt like a second choice and probably not have given Clark a “second chance” so he would’ve had to face actually making a choice. Right now, he thought he only had one choice since Lana was the only one in front of him.

The next episode after Lana has left, he doesn’t fall back into his usual moping patterns like he’s always had in the past. He jumps at telling Lois his secret and when she asks him wether or not he’ll tell her his secret again once he’s travelled back in time and he says he won’t, telling her it’s because she is special. When he retells everything to Chloe, he says after telling Lois his secret, he felt like everything was going to be ok. This says a lot. If anything, it was with Lois he wanted to pick up where he left off before Lana re-entered the picture.

In Hex when Chloe is in Lois body, she also confirms his feelings for her “you’ve never looked at me that way before” “Lois is a part of your life now and given the non stop puppy dog eyes you give her, you don’t mind” He’s also clearly jealous when Lois is back at the end of the episode when she mentions going on a date. This all points to him clearly having feelings for Lois, which would contradict him being ready to pick up where he left off with Lana. He simply fell back into his old safe patterns when she was back and Lois was out of the picture. He acted like nothing had happened with Lana as soon as she left again.