[RPG] Is the additional effect of Radiance of the Dawn dependent on the first effect

There are some guidelines to help a DM decide what category a creature falls under so let's take a look at page 185 of the PHB:

In general terms, an NPC’s attitude toward you is described as friendly, indifferent, or hostile. Friendly NPCs are predisposed to help you, and hostile ones are inclined to get in your way.

Page 244 of the DMG goes into a little more detail but there's too much to quote it all here so I'll only post the relevant parts.

• A friendly creature wants to help the adventurers and wishes for them to succeed.
• An indifferent creature might help or hinder the party, depending on what the creature sees as most beneficial.
• A hostile creature opposes the adventurers and their goals but doesn't necessarily attack them on sight.

In general it's up to the DM to use their best judgement to determine which category a creature falls into and whether or not they are affected by certain spells.

If the covert spy in the party works for someone who "opposes the adventurers and their goals" then they might also technically count as hostile. However this is definitely a case where the rules aren't strictly defined and the DM can rule however they like. If the DM doesn't want to "out" the spy and the party thinks he's their friend, it would make sense to rule that something like Aura of Life would work on him. But if the DM wants to reveal the spy for what they really are or at least hint at it than you would be perfectly fine to rule otherwise.

As for whether an invisible creature can be affect by such spells; a creature's visibility has nothing to do with their attitude. If it's hostile when visible, it's hostile when invisible, whether the party knows it's there or not. As such, we still follow the general spellcasting rules regarding invisibility and spells and if a hostile invisible creature happens to be in range of Radiance of Dawn when it's cast, it would still be affected.

No

Strict limitations like that are told to you in the ability's description. Since the ability does not say it must be used outside, it doesn't have to be.

Additionally, there is substantial evidence that the designers already had experience with requiring or taking into account environmental conditions when designing spells and effects. The Call Lightning spell allows you to take control of an existing storm if you are outside in stormy weather. Additionally, the Control Weather spell requires you to be outside and maintain a clear path to the sky.

Since the limit for being in a clear view of the sky is already used in the Control Weather spell, and since the Call Lightning spell has a special condition when you are outside, we know for a fact that the designers were aware of the effect of the environment when casting spells. Since Radiance of the Dawn does not list any requirements for being outside, and because these limitations exist in other places, it is reasonable to assume that the designers would have listed a requirement on Radiance of the Dawn if one were intended. No such requirement is listed, and thus we can reasonably assume that no limit in terms of being outside was intended.

As a thematic parallel, there are two spells in the PHB, Sunburst and Sunbeam that can actually create sunlight, neither of which requires you to be outside. Since we know that sunlight can be magically generated, it isn't beyond belief that a Light Domain Cleric would have the power to harness sunlight at will.