It appears the ERJ 190 has about 6.5-7.0 endurance. The flight from CPT to HLE is about 4.5 hours which leaves at least 2 hours for island reserves. Going back 1100 nm to Africa seems like a stretch. St, Helena just issued a RFP for tender offers:
http://www.sainthelena.gov.sh/wp-con...l-document.pdf