Yes, D7 has wireless router mode and it can work with another modem. http://www.tp-link.com/resources/simulator/Archer_D7/index.htm

Maybe their function are similar to each other. BUT Chinese products cannot upgrade Enghlish firmware of English products, and vice versa. If you need installation assitance, try to email to...

Did you connect only one PC to WR841N and test what happens? Try to change DNS(DHCP---DHCP Settings) to 8.8.8.8 and 8.8.4.4.

Try to reduce MTU size. http://www.tp-link.com/en/faq-69.html

A little confused... WR740N is working as a wireless router instead of an access point with DHCP disabled, right? Try to configure IP&MAC Binding on web interface of WR740N.

No driver for Windows 10. And Win 10 has not been released officially. Did you try driver for Win8 or Win 8.1.