The best thing to do is call me at 800-672-3274.
To arrive Kilimanjaro Airport on Sunday morning, March 29, the latest you could leave Europe would be Saturday evening, March 28. You could also leave Europe Saturday morning on a couple day-time flights to Nairobi, which then connect immediately to Tanzania, arriving Saturday night late. We would then overnight you in the comfortable KIA Lodge adjacent the airport and pick you up Sunday morning as planned.
The above means the latest you can leave home is Friday night, March 27.
The earliest you can return home is sometime in the afternoon, Friday, April 10.
As of September 4, the cheapest roundtrip fare in economy from New York was $1364.10. The cheapest roundtrip fare from Europe was $1090.60, which means that use of frequent flyer miles only as far as Europe will not save you very much. But if you have 75-80,000 miles, you might get a roundtrip all the way to Nairobi.