@malavkarkar has answered the first part.If anyone can confirm that my friend can transfer points from Amex to me, it would be very helpful.
Also, If anyone can confirm that if I purchase points using the HDFC Marriot Bonvoy Card, do I get a 5x bonus for using it on Marriot?
No you won’t get 5X now (Oct/Nov) or anytime else.
Points are purchased via points.com even when you go via Marriott.com.