I realize this film is not to be taken too seriously, but at the beginning, after the time trip, how on earth did the four of them get to the top of the mountain without passing through the ski lodge or seeing anyone else?
And they were all naked in the hot tub, where did they get their clothes/ski gear/etc without noticing their hotel room and things had changed?
Well, figure they were heavily hung over in the morning and possibly got up very early. If we assume that their clothes and gear were not identical but probably very similar, and they wound up getting up fairly early in the morning, they may have stumbled through getting dressed, gotten through a mostly empty lodge without looking at anyone very closely or talking to them other than a few vague mutters that wouldn't have had anything to do with the year ("Four tickets." "Five dollars, please." "Hnnh. (Only that much?)"), and so on. Basically it's only once they got up to the top that they started to be coherent enough to notice things.
In the past, the characters look like their older selves to each other but like their younger selves to everybody else. When they leave Lou back in the past should he not look like a 60 year old to the other characters in the modern day?
What was the last 20 years in the new timeline like from Lou's perspective?
When Adam, Nick and Jacob get back to the present, a lot of things are different but their family members and coworkers recognise them. Does that mean there were alternate versions of the characters walking around for 20 years who lost their memories in 2010?
I assumed that they would regain their memories of the "new" past after a while, or else that ending is not particularly happy, when you think about it.