Can I add LL to an existing save?

Generally yes, although there are likely to be more bugs and edge-cases if you do this, as it won't have been well tested. If you have already started producing processing units in your save, then you will not be getting the intended progression.

How does LL compare to Space Age?

Lunar Landings shares a similar design philosophy to Space Age in that it adds a specific named planet, with its own challenges and limitations, and is a natural extension of vanilla gameplay. LL is much shorter than SA. Cross-surface item transfer is done by launching rockets (inserter-loaded and generally single-item) directly at named landing pads. LL and SA will be compatible in the future.

Will LL be playable with other planet mods?

When LL is being played without SA, it is free to act like an overhaul and change vanilla balance to suit the intended LL gameplay. However, if this same philosophy is applied to LL+SA, then those same changes will now have many knock-on effects when considering balance on other planets, especially since it needs to be compatible with other planet mods (like Maraxsis).
So, LL when played with SA will try to make minimal changes to gameplay/balance on other planets, and solely function as a add-on/rework of the Nauvis part of the game. It will probably still be more 'invasive' than your average planet mod because it is so tightly integrated into the base progression, but my intention is that it still manages to play nice with other planet mods that aren't themselves modifying Nauvis progression.

What will Space Age compatibility be like?

Luna will be kept separate from the SA orbital logistics system. Space platforms cannot enter Luna orbit, and items cannot be launched directly from Luna to a space platform.

After unlocking red circuits (and before unlocking space platforms), you'll unlock LL's smaller, reusable rocket silos. These function much more like base rocket silos: they must be loaded by inserter, will only launch to the destination landing pad specified in its interface, and can only ever send items to and from Luna. You must then progress on Luna to unlock silicon, which in turn unlocks processing units which can be shipped back to Nauvis. (At this stage, processing units can only be crafted on Luna, using silicon, although the recipe uses less copper than standard). From here, you can unlock regular SA rocket silos, build SA rockets with the processing units, and start building space platforms and automating space science on them.

[Maybe: Before unlocking space platform thrusters, you'll need to return to Luna to complete it's first science pack (which was previously called Space science, but now will be called Astral/Gravitronic/Gravimetric (?) science), crafted from aluminium and telescope data.]

Just before unlocking other planets, you'll unlock a new processing unit recipe which doesn't require silicon (it will be identical to the standard SA recipe). This allows you to craft processing units as usual on new planets. [Maybe: You can't use this recipe on Nauvis or Luna though]. You'll also be able to use low gravity assemblers on space platforms, and extract silicon from asteroids.

Before unlocking quantum processors on Aquilo, you'll have to complete Luna's second science pack (Quantum science), by crafting quantum circuits (previously called quantum processors, but Aquilo also has an item called that). Luna's quantum circuits will be an ingredient in Aquilo's quantum processor recipe.

SA endgame will stay unchanged - LL's interstellar rocket silo will not appear.

Will I be able to add SA to my LL save once LL+SA is released?

No.

Will I be able to add LL to my SA save once LL+SA is released?

Generally yes, although there are likely to be more bugs and edge-cases if you do this, as it won't have been well tested. If you have already started producing processing units in your save, then you will not be getting the intended progression.

How does LL compare to Space Exploration?

Lunar Landings is a completely standalone mod. Some parts may look similar because it uses some graphics from SE, and since both mods involve building on other planets, there is some gameplay overlap.
Lunar Landings is significantly shorter and easier. No changes are made to vanilla until chemical science, and only 2 new science packs are added (compared to SE's 22).