As per swisstemples' recommendations and his own personal use:
1) He does both, you can do either. Oral is more effective because the absorption with topical isn't complete, you'd need to mix it with DMSO unless you keep it on your scalp for 8+ hours. However, he does mention small amounts of castor oil (fingertip amounts or so) rubbed into the temples will completely absorb, so that may be worth trying if you're adamant about topical use.
2) Don't get Jamaican black castor oil (it contains other parts of the castor bean which are unnecessary), get cold-pressed 100% castor oil, it should look clear and pale yellow. Make sure it's either food grade or pharmaceutical grade (USP or BP) so you guarantee you're getting the standardised 90% ricinoleic acid content (the active ingredient in castor oil that activates EP3 and EP4 prostanoid receptors). Also some grades of castor oil aren't fit for internal ingestion so adhering to the aforementioned standards is imperative if you're planning on taking it orally. Brands I can recommend are Now Foods Castor Oil or any castor oil that is certified by USP or BP.
The normal oral dosage is a sub-laxative dose of 1-2 mL. Swisstemples recommends 1 mL as he says 2 mL causes more hypertrichotic side effects (growing vellus hairs on your forehead and below the eyes, darkening of scalp and body hair, increased body hair growth). That being said these side effects go away after cessation of treatment, any unwanted facial hair growth will fall out while still keeping your scalp hair gains (as PGE2 related pathways aren't dependant on the medication that grew them providing you are on an anti-androgen and/or setipiprant for maintenance)
I plan on implementing it myself in the coming months (the wait is due to wanting to see if results are present that can be attributed to my current regimen), most likely only orally. I'll post updates in My Story thread pertaining to this endeavour.