If you're actually wanting a one-shot, as in just one session, you'll need way less stuff than you think. It'll vary depending on your players of course, but for a four hour session with my players, I think three encounters (with only one of them being combat) would be about right. My "one shots" always end up being "some shots" of 4-5 sessions.

Same remarks as Boatswain, the most important in a one-shot is to reduce the number of plot points in your story, because players will deviate from it and spend a hell lot of time on what you thought were details. Gladly, your scenario is quite simple : going from point A to point B on the road, it's hard to derail. By the way, make them start on the road, or those "details where they spend time" will be half the session spent in Phandalin shopping to prepare for the trip.

Regarding having the goblin as a DMPC : it's perfectly fine to have an NPC following the party and even fighting with them, you just have to make sure they never overshine the party. Make them a few level lower, and play them in a support role, never let them have the kill or solve a problem by themselves. And when the players will start asking questions to the NPC thinking they're the voice of the DM and know everything, make them say stupid things or obvious errors, explaining, as the DM, that the NPC doesn't seem to know better. From there, you will be fine.

Note: this is no general one-shot advice, but given the context you mentioned : keep in mind that the characters your players will create for your one-shot will then be part of your campaign.