When, and can they get there ahead of the Iberians? What would motivate them to start sailing out that far, since they didn't OTL?
As for the specific questions:
1. I am of the opinion that the fate of Guanches will meet or exceed every horror of their fate after Iberians came to conquer and enslave them, because Morocco was a remarkably active and hungry slaver state since the middle ages and until a very late era, and a nearby pagan population would be of high priority for the slave trade. Moroccans have all the Eurasian diseases, as well. The Guanches do not.
2. I am likewise of the opinion that provided you have something resembling a caravel, and a base in Madeira, you will eventually happen upon the Volta do Mar; if you frequently put in at the Canaries also, you will discover the Summer Easterlies. It is very hard to never accidentally drift out to America after that. But that's just my opinion.