Diploma

Yes, you can get admission in COEP with 85% in Diploma.
The minimum eligibility criteria for admission to B.Tech.(lateral entry) at COEP - College of Engineering is that a candidate must have completed Diploma in Engineering and Technology in relevant stream with a minimum of 45% marks from an All India Council for Technical Education or Central or state government approved institution or its equivalent.

Yes, you can surely take admission in aeronautical engineering after completing Diploma in mechanical engineering.

With 10 +2, you can apply for this course.
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As you have completed your Diploma in Mechanical Engineering , you can apply for Aircraft Maintenance Engineering Or you can apply for BSc in Aeronautics at HAL , Ojhar , Nashik !

Yes, you can do 11th and 12th PUC. ITI courses, Other vocational courses, Certificate courses, Diploma in Vocation, Domain Skilling programs, Skill Development programs. Or many other courses.

Please save yourself from distance courses in engineering. Such courses are not recognized. AICTE doesn't recognize such courses. Regards.

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Well the cutoff varies according to branch. But considering overall factor 90%minimum is required in Diploma for any chances in COEP irrespective of category.90% is also not sure rest everything will depend on luck and results of overall batch in state. Approximately cutoff is like 94 for Mech.93 Civil.90Metallurgy.92 Entc.94 IT. For Open category.