visa rwanda and visa brundi coming from rwanda

hi there,
i'm austrian passport-holder, leaving to rwanda middle of february and there's a few questions, hope anyone can answer them:
1. i'm trying to apply for an online visa but as it seems i need an invitation letter. as i have friends there i don't have a hotel reservation (which should be sufficient i guess). does anybody know if i can just send an "invitation-letter" from my friends?
2. we want to go to burundi as well, as far as i know it should be fine if i apply for the visa in kigali, is that right?
3. is there any possibility to get a multiple visa upon arrival for rwanda or will i have to get a new one when i re-enter?
4. has anybody recently been to the DRC? is it okay to go there in a group or is it just a struggle?
5. and last: is it better to take euro or US dollar? in tanzania the changing rate for dollar was the same as for euro...
austria doesn't have an embassy neither for rwanda nor for burundi, i can't send my passport to the embassy in berlin as i'm leaving to beirut in 2 weeks...
thanks!