1 day in Interlaken - need info

Is there a way to head to Lauterbrunnen from "Grindewald FIRST" without coming back to Interlaken?

Yes, change trains at Zwei Luetschinen. You can also go "over the top" by taking the Wengneralpbahn up to Kleine Scheidegg and then down the other side through Wengen to Lauterbrunnen. Will be more expensive and probably take longer but be very scenic.

Seems to be a very hectic schedule. Any reason to be back in Interlaken by 8? Another train to another mountain paradise?

Last question first. Yes, the local railway connects Lauterbrunnen to Grindelwald via Wengen and Kleine Scheidegg without needing to go back to Interlaken.

Suggestion on your one-day itinerary. Skip Harder Klum, and go straight from Interlaken to Lauterbrunnen. If it's clear, head up the Schilthorn. Depending on weather and how much time/energy you have, you can also check out Murren, explore sights in the Lauterbrunnen valley (waterfalls), head up the other side of the valley through Wengen and over Kleine Scheidegg and down to Grindelwald, etc.

Regarding returning to Interlaken, unless you have already booked lodging or need to get back to catch a train to keep your schedule, there's no reason to return to Interlaken that evening. For a better Swiss Alp experience, find a place in Murren, Wengen, Lauterbrunnen or even Grindelwald.

Do not waste your vacation time in the town of Interlaken which is NOT in the Swiss Alps. You obviously came to see those stunning alps because you want to see Grindelwald, Schilthron, First, etc.

You'll have to connect at Interlaken Ost to the little train that will take you to Lauterbrunnen. The Lauterbrunnen Valley sits in the middle of the things you want to see and the stunning scenery. Why waste time trekking back to Interlaken when you could just stay in Lauterbrunnen or one of the other alpine villages.

Interlaken is just a town on your way to see the alps. Skip it and choose a better overnight location which IS in the alps.

Don't worry about Interlaken Swap for just 1 day/night its fine. You've got excellent transport link to the attractions you want to visit. In addition food options in Interlaken are more varied, and cheaper than the mountain villages.
I've had good stays at this place neat the West Station:

http://www.lotschberg.ch/englisch/hotel-bnb-interlaken

The reason I need to get back to Interlaken is because we have a train to catch back to Bern.

The last train to Bern leaves at 11 pm, so no rush. One every hour.

Will Swiss pass cover the Train fare from Lauterbrunnen to Zwei Luetschinen & from Zwei Luetschinen to Grindlewald?
I will be having a 4 day swiss pass.

Yes it will. It would not cover the full cost of using the Wengneralpbahn, which takes an hour longer. It covers the Schilthornbahn from Muerren to Schilthorn at 50%.

Yes, that route to Grindelwald is fully covered by the Swiss Travel Pass but moving on from Grindelwald to Kleine Scheidegg is not. The map that comes with your pass shows covered versus discounted routes. It's also online at http://www.swisstravelsystem.com/en/home.html (see the small link).

Swap: Of course you can stay in any village you want (including touristy trinkety Interlaken), but just so that you understand...the Swiss Transportation System is all integrated. When you say you must get back to Interlaken because you have a train to catch for Bern - do you realize that you can leave from ANY of the mountain villages and get to Bern? Yes, the train you take will originate for example in Grindelwald or Lauterbrunnen or Murren or Wengen and will stop at the Interlaken Ost station for passengers to depart to make connections to other trains like to Bern (for example). Just because you have to catch a train to Bern doesn't mean you have to spend the night in Interlaken. Of course it's fine you decide to do that, but it's totally unnecessary.

Hotels cheaper in Interlaken - obviously that comment came from someone who never stayed at the Chalet Fontana in Murren.

The OP says he is staying in Bern. This entire schedule is a day trip.

i completelyagree with hubestur. I would stay in wengen and travel to Klein or Murren or Lauterbrunnen. Its a wonderful small village