Don't forget that the Redbrand encounter is triggered by the players visiting Phandalin for the first time. They are more than likely to meet NPCs in the tavern who will allow you to seed interest in sorting out the bandits. 

Use Sildar if he survived, he will wonder why his contact is not in Phandalin and may need the party's help in tracking him down.

Combat may prove to be tricky but there's nothing to suggest that they can't plan a stealth mission to rescue Gundren. 

And finally,  Phandalin isn't going away so you could still explore the sidequests after they get back from Cragmaw Castle! 

On top of the above, maybe add the guy they're supposed to deliver the cart to to those taken by the Redbrands with the woodcutters family as well. They're a kind of threat in the town and can give you a chance to stall them if they're held up there before they try to head out to Cragmaw. It only takes dealing with the Redbrand hideout to get them up to level 3 which would still make Cragmaw tricky but they'll be roughly where they should be.

Welcome to DMing! 

In the beginning I think a lot of us what the feeling of "control" in our game, so we can "feel prepared" for what the characters are going to do.

In this case, I don't think you have much to worry about, as you can have fun both ways. When the party enters town, make sure they realize there is a "redbrand threat". You can even make it personal, buy having the redbrands  be actively harassing townfolk as the party enters.    Then the party has a decision to make, "deal with redbrands" or "go find gundren".  I also wouldn't change the threat level in Cragmaw much,  the party could be defeated (or have 1 or 2 captured) then they REALLY have decisions to make.   Also, if they see the difficulty is high they can then choose to deal with something else (ie the redbrands or other side "quests").

Don't be afraid to let things grow organically.  I recommend just knowing your villains and their motivations and what/where they are.  Then let your party's interactions with the world really flesh it out.  Put players in positions to make interesting choices, even if it doesn't always work well, they will remember the story. Ie "remember when Sue was playing Gimlet the dwarf and she charged the goblin castle by herself?"

I would say don't adjust the level of the Cragmaw Castle, if they want to go straight for it, it will be difficult.  In honesty depending on how they approach the castle it might not be that difficult of a fight, they could easily come in the back way and only have a couple enemies to deal with.  Or if you are feeling forgiving let Sildar go with them.  Perhaps something like his focus is on his dwarf friend, but he might drag the group back to Phandalin after you find the dwarf to deal with searching for his missing contact.

Other ideas to give them incentive to go to some sidequesty areas, either after fighting at Cragmaw they meet a druid who has been tracking the goblins and hobgoblins in the area tells them of a dragon moving into an abandoned village, or on their way to the castle, he warns them that they are not prepared for what lies ahead and might redirect them to Thundertree or something first (there you might have to change the health on the dragon).