It Isn't Based in History

You won't find the answer to this element of your question in that form:

... rather an outside source that shows the historical connection) to show the origin of scimitar as a druid weapon in Dungeons and Dragons, and if, as I suspect, the scimitar was a substitute for a curved sword thought to be used by a historical druid.

That isn't how the scimitar arrived as a standard Druid weapon.

For Dungeons and Dragons, the original source of the druid and his weapons is in Supplement 3 to Original D&D, Eldritch Wizardry. On page 2, it limits Druids to the following weapons:

Daggers, sickles, or crescent shaped swords, spears, slings, and oil.

Most of us playing druids¹ turned to the only curved sword we were familiar with, a scimitar. That bit of expediency was formalized in AD&D 1st edition PHB (p. 19, Table II, Permitted Weapons) and became an established trope/feature.

The relationship between the historical sickle² used to harvest misteltoe and a curved sword was a stylistic point. From the man himself:(note:Gary was posting as Col_Pladoh)

Quote Originally Posted by Heathansson
Hello, Colonel! Hope you're feeling better!!! Just a question that stretches back down the eons to 1e.: why do druids use scimitars? It just seems curious with the Celtic connection.

Heh, It is because the scimitar is as close a sword weapon I could come up with to match the druids' mistletoe-harvesting sickle. Cheers, Gary

Druids as a character class were originally a sub-class of Cleric. At that point in the game's evolution, Clerics generally could not use edged weapons. The scimitar was a stark example of how different a Druid was from the garden variety Cleric. The "curved sword" theme related to the sickle was carried over in Unearthed Arcana (1985) when the khopesh sword for druids was added. (Historical evidence for relationships between Druids and Ancient Egypt: a topic for another question?)

Use of the scimitar / curved sword by Druids is not supported in history, per your point in the question (this squares with the modest reading/research I have done on the topic). Many of the OD&D rules, standards, and concepts came from short stories, books, movies, mythology and lore ... often at the expense of historical accuracy.

The Druid began as an encounter/monster in Greyhawk. He led uncivilized / barbarian / berserker soldiers. This comes off as a variation on how the Romans viewed the Celtic warrior bands, put into game form. It is worth remembering how Raw and Unpolished the OD&D material is, and to a certain extent AD&D 1e. A point made by Gary Gygax regarding Druids as a game feature ...

The primary appeal of the Druid class from a creative standpoint is that the Romans were so thorough in destroying them and their religion that we know virtually nothing about either

¹ Personal note: I first played a druid in D&D in 1976 who was more inspired by Getafix, the Druid from Asterix and Obelix cartoons, than by any Celtic spiritual leader/wise man in Roman and pre-Roman Britain. I learned about that later.

² Pliny recorded the use of using golden sickles to harvest mistletoe. Book 16: The Natural History of the Forest Trees. The Druids--for that is the name they give to their magicians -- held nothing more sacred than the mistletoe and the tree that bears it, supposing always that tree to be the robur. The mistletoe ... when found, is gathered ... fifth day of the moon ... Clad in a white robe ... cuts the mistletoe with a golden sickle.

The pros of Scorching Ray are:

• Assuming no attacks miss, it does more damage than Guiding Bolt.
• You have the option to attack multiple targets when using Scorching Ray.
• Eventually, when you have higher level spell slots, it will scale much better than Guiding Bolt.
• Scorching Ray works with the Elemental Adept feat - which allows you to ignore fire resistance and raises the floor on your minimum damage. (Credit - yinzanat)
• Against enemy spellcasters, Scorching Ray provides more opportunities to break their concentration. (Credit - gburton)

The cons of Scorching Ray are:

• It deals fire damage instead of radiant damage. While this is situational, there are more monsters in the Monster Manual that resist fire than resist radiant.
• It does not grant advantage to the next attack roll against your target.
• If you have advantage for being hidden it will not apply to your second and third attack.

The thing that could be either, depending on how superstitious you are about your dice (and what the AC of your target is):

• Scorching Ray makes three separate attack rolls.

You would want to use Scorching Ray if:

• There are multiple targets that you are confident in your ability to hit.
• You are fighting something that is vulnerable to fire damage.
• You need to deal more damage right now.

You would want to use Guiding Bolt if:

• You need to set an ally up for an upcoming attack against the same target.
• Your allies are having a hard time landing attacks against the target.
• Your next ally to attack is a rogue who will get sneak attack damage from the advantage. (Credit - DaleM)
• There is only one target. (You may wish also just not upcasting Guiding Bolt in this scenario and relying on your party to use the advantage.)
• You have advantage on this attack (ie you are hidden).