What makes Lone Druid "work"?

Lone Druid is generally played as an offlane hero, this is because he's hard to catch out since his  Spirit Bear can farm while he stands behind somewhere safe.

During the laning phase, do Lone Druid players have both units selected to last hit the same creeps, or do they use both units to attack individual creeps?

Usually doesn't matter the method, as long as your can get those last hits in the lane, what I do usually is send the Spirit Bear to harass and zone while i use the hero to last hit early on. When you have decent points into Synergy, the Spirit Bear gets really good damage especially with Quelling Blade, from there i'd play my hero more defensively to not die, or harass with my hero and farm with my  Spirit Bear. I wouldnt attack individual creeps or keep auto attacking, you end up pushing out the lane too much, leaving you exposed to ganks.

What makes Lone Druid "work"?

Both Lone Druid and his Spirit Bear have mediocre to low base damage, even with Synergy. I understand that Spirit Bear can use items, but carries that have actual skills can also do so. Is it Roots or something?

Lone Druid works as a carry that has a very safe play style since you let you Spirit Bear do the hitting, while your hero turns on true form to become tanky and keeps out of the fight for the most part.

Usually you build some boots on your hero and all the other items on your  Spirit Bear, you Spirit Bear has 2700 hp at level 7 as well as the ability to Demolish buildings very quickly making him a very good split pusher. Items like Radiance or Hand of Midas is very common since it helps you flash farm to bring your Spirit Bear and yourself to 12 slots of items. With the Spirit Bear being so tanky, he also dishes out tons of damage with Radiance. With max Synergy and Rabid, the Spirit Bear moves at 522 MS with phase, its very good at chasing enemies in teamfights and easily kills them if the root procs.

Be careful though as in the late game enemies with high right click can kill the Spirit Bear easily, and also it gives 300gold throughout the game. May be hard to to contol the Spirit Bear perfectly at first~

Items like Assault Cuirass, Skull Basher, Mjollnir are really good on the Spirit Bear too

• Spirit Bear doesn't benefit from stats but it needs attack speed and damage. That is why items like Radiance, Assault Cuirass, Maelstrom is recommended.
• Spirit Bear also doesn't have mana. For example, by default you can not use active of Mjollnir with  Spirit Bear. Though you can buy an item that grants mana for Spirit Bear and use Mjollnir that way. For this reason Maelstrom + Hyperstone in Spirit Bear might be more preferable than Mjollnir.
• You can think Lone Druid as a support for your carry  Spirit Bear. You can get items like Vladmir's Offering on Lone Druid to support the Spirit Bear for example.
• Trying to last hit on same creep with your Spirit Bear and hero is probably easier, less chance to miss. I think this is good in the beginning, but this might be not ideal for high levels.
• Also don't forget you can use your Spirit Bear as your personal courier or scouting or if you have Radiance split push a lane.
• You can (kind of) micro your Spirit Bear with Ctrl commands without using control groups. For example, if you issue a move command on your hero by holding Ctrl, this moves your hero and all his summons.
• Since Lone Druid has effectively 12 slots, some item choices becomes viable. You don't always have to build full recipes. You can hold on casual base items longer than other heroes, if you choose so.