Doing a loop would add miles, but only 2-300 miles depending on your route choice and would no equate to much [5-6 hours] over 14-18 days, but as mentioned previously it's your trip and you base it on your preferences.

You can get to Yosemite from LV in the same day, but I would ask why ? You have plenty of time and you would be rushing through some wonderful scenery to do so. The journey is as pleasurable as the destination !

If I were to choose from the above 2 options it would most likely be option 2 with that overnight on route from LV to Yosemite with a trip through Death Valley.

If it were I, I would include the GC and take the loop. That's my preference and doesn't represent 'best' as that is another term like 'must see' we don't 'do' here. It's all about the individual and that's the beauty of planning your own road trip. We are here to help in the best way we can but ultimately you should go with your goals in mind. For example with 18 days I would be looking at parks in Southern Utah like Bryce and Zion, the Grand canyon and maybe Monument valley and 'up' the pace a little, but that's not going to suit everyone and is another story !