Can dioxycycline (for 70days, trip to india) interfere with the effectiveness of my bc (yaz)? Back-up contraception advised?

Yes. Doxycycline, like many antibiotics, are preferentially absorbed when taken at the same time as an oral contraceptive pill decreasing the absorption of the birth control pill and therefore its' effectiveness. Space the Yaz (drospirenone and ethinyl estradiol) pill and the antibiotic 12 hours apart, and consider a "back up" means of contraception during the 70 days of the doxycycline.
Should be ok. The data suggests that not all women taking the pill will have an increased risk for pregnancy while on doxycycline, but that some do, and it's hard to guess which women those are. You should definitely consider using condoms as a back-up method of contraception to stay safe!
Yes it could. You should use back up birth control. You should also be aware that Doxycycline significantly increases your susceptibility to sunburn.
Not interfere. Doxycycline does not appear to reduce the effectiveness of oral contraceptives as previously thought. Studies have shown that people taking doxcycline long term along with oral contraceptives do not have a higher rate of pregnancy. Of course, oral contraceptives only prevent pregnancy, you will want to use a condom for protection against infections. Doxycyline is an excellent, affordable way to prevent malaria. Be sure to take it every day, preferably at the same time each day and don't forget insect repellant. Be sure to let your doctor know you are taking both medications.