You won't like some of these answers...

1. Ask your table and your player what they want

   • Ask the table openly. "Bob just rolled really bad stats, and I don't want to hold him up to that. Would you guys think it's fair if we let him re-roll?"

   • Ask the player directly. "I'm going to find it really hard to tailor encounters for you, because your ability scores are so low. Do you think you'll have fun? What would you want to do if you could fix it?"

   • And then, most importantly, listen to their answers. They'll probably tell you exactly what you should do.

2. Tailor encounters such that ability scores are not important

   • Puzzles, social interactions that do not involve rolling, things that can be solved by spells that do not have saving throws, attack rolls, or ability checks -- such as Detect Thoughts, Minor Illusion, Comprehend Languages, etc -- will go a long way to giving utility to a commoner-type character. If you place the burden of the encounters on the players, not the characters, then you are giving everyone, including the low-stats character, an equal chance.

   • Of course, this cannot be done in every encounter. Sometimes, combat will happen. And that character may not have too much utility then.

3. Allow them to change classes into a pure support type

   • Let them be a cleric, wizard, or druid. A cleric can spend their turn putting up Bless and Guidance, which do not require ability checks to cast. Wizards could eventually cast Haste on their allies. Druids can Wildshape into any beast, replacing their physical scores entirely and hiding their real stats behind class abilities. Meanwhile they can spend ASIs improving their Wisdom score.

4. Give them magic items

   • This might (not necessarily will) feel unfair to the other players, but give them magic items to make up for their deficiencies. The issue here is, he is getting rewarded for rolling low, and so the other players might feel punished for rolling high/average.

5. Re-roll their stats or have them use point buy

   • If you are the DM, then it is within your rights to implement this solution. Put a narrative spin to it like their inner spirit has awakened due to intense trauma/stress.

   • If you are not the DM, and you are not being allowed to roll, express your concerns to the DM that this is not fun for you. Most DMs will listen to their players if they express their concerns.

6. Kill them off

   • An extreme solution, I'll admit, but if you as the DM don't want to re-roll stats, then kill this commoner adventurer who's tagging along with the party. People like that are simply not cut out to be an adventurer, and death is a part of the risk of adventuring. Now that he's dead, you can have them roll up new stats for a new character.

   • If you are the player, I'd hate to say kill your character... but kill your character. If you are truly unhappy with it, then you cannot and should not try to address the problem by tacking on band-aid solutions to it. It's not worth the stress and tension to keep dedicating your time to playing something you're not enjoying.

You cannot estimate how long an adventure will take.

In my experience, it is basically impossible to make any sort of precise, accurate estimate regarding how long any given adventure will take. While it may be possible to get in the ballpark (a single encounter isn't going to take 4 hours, and a huge campaign isn't going to take a single session), there will almost always be unexpected externalities that cause you to go way over or way under on your estimates.

For example, I once prepped a multi-hour encounter with a camp of bandits, only to have the PCs agree to pay off the bandits and skip the entire encounter. In another case, I've played in a game where we found a cave, said "nope," and left, and instead went to harass a random NPC for a while.

In your example, it's possible that the character introduction section could take 5 minutes ("My name is [x], I'm from [y]") or it could take the whole session. Then, the initial goblin ambush can be quite deadly, especially if you're using surprise rules--the session can end there, if your players are unlucky. As your players encounter the sentry goblins in the guard post, they could engage in a protracted combat, or bypass the encounter by sneaking through, or burn the whole thing down.

As you can see, the breadth of options that PCs have makes it very hard to estimate how long it will take for them to get through any particular section of the adventure.

All it can take for you to wildly off in your estimates is for one player to decide that they feel like doing something different at any given moment. Maybe instead of charging into battle like they have every time, the barbarian decides to be a bit more cautious or talkative, for instance.

Breakpoints are everywhere

Generally, I find it perfectly ok to have a breakpoint anywhere that's not in the middle of a combat encounter. For example, the book breaks down a number of encounters in sequence: the goblin blind, the wolves, the steep passage, and so forth. While the party's experience through the dungeon is continuous, it's really organized by different scenes. Ending the session right before or after these is a natural breakpoint, since each is a scene transition.

Also, your players might not care about "natural" breakpoints as much as you do. Speaking for myself, I'd rather have a session end when I have to go home instead of being forced to stick around for extra time just for plot reasons. I'd ask your players how much they care about natural breakpoints versus going home on time--you might be surprised at their responses.