Diploma

Yes, you can pursue it after completing your 10th.

It's impossible as it is your final year it's only possible up to the 2nd year.

For Government Institute of Hotel Management, you can take direct admission for Diploma but you are eligible as per their terms and conditions. It includes English subject, age, and 12th pass mark sheet. And there is no entrance exam, but a counselling session held in the institute for admission.

As far as my knowledge goes, AIILSG doesn't provide SI course in correspondence mode, but only regular courses or holiday batches (i.e. Saturday, Sunday and other holidays).

You need to give the NATA exam (preferably before the end of May) and then in the month of June-July apply for the admission process of DTE. If your NATA score and 12th percentage are really good then you might get into Sir J.J. college of Architecturing.
Admission to B.Arch. is done on the basis of

No Diploma holders are not entertained for CPL Training. You will have to clear class 12 with 50% marks in PCM. This is the only way by which you can apply for CPL Training. Diploma does not holds good in terms of DGCA regulations.

Cutoff of 2016 in TEAMCET for admissions in JNTU
Chemical Engineering
Closing Rank - 30072
Civil Engineering
Closing Rank - 12527
Computer Science & Engineering
Closing Rank - 8487
Electronics & Communication Engineering
Closing Rank - 10535
Electrical & Electronics Engineering
Closing Rank - 11425
Mechanical

The candidate can expect a starting salary of 25-30 thousand per month. Additional bonus and retention bonus are added at the end of each year, which is calculated depending on the performance of the candidate.