Have you read any of the novels set in Ravnica? Though I haven't read them, I suspect that they would be full of useful details you couldn't get from cards alone.
I would like peoples opinion on what alignments each of the 10 guilds are, some are extremely easy while others due to their fluff are kinda awkward.
If at all possible, try to have at least one guild for each of the 9 alignments. This will give all sorts of people a place in Ravnican society, even if the alignments have to be fudged a little.
Divine magic is kinda tricky on Ravnica because there are hardly any beings that can be considered divine. I dont want to leave out divine casters from the setting and am looking for a way to incorporate them that would not directly contradict the fluff of the setting.
There are 18 Cleric creature cards among the 6 Ravnica expansions. The connection between the creature type Cleric and the class Cleric is tenuous at best, but looking at these cards should give you some indications of how divine-style magic works in Ravnica. Most of them work for a guild, but a few, like Caregiver and Urbis Protector, are described in their flavor text as not belonging to any guild. Of the 18 clerics, 7 have an Orzhov watermark, 2 have a Selesnya watermark, 3 have a Rakdos watermark, 2 have a Boros watermark, and 4 have no watermark. Divine spellcasters in other guilds use different creature types, which should give you some idea of how Ravnicans themselves think of divine magic.
In general, clerics in MtG gain spellcasting from faith, rather than from a higher power itself. In D&D, clerics of ideals fit this flavor perfectly. If you want something more specific for cleric characters, you might decide that each guild has a few domains which are especially common among its clerics.
Since it is too hard to resist, the campaign will be a gestalt game. It simply makes too much sense for a campaign set in Ravnica to be gestalt.
If you came up with color associations for each class (Barbarian is R, cleric is W/B, druid is G, paladin is W, etc.), you could encourage players to have different colors on the two sides of the gestalt. Depending on your group, this might feel like a cool reflection of multicolored cards, or it might feel like a stupid gimmick; in your place I would use it as a suggestion, not a requirement.