Mr. Khan,
1. The HSMP Team will accept your application as an under 28 category if it is received before your 28th birthday. Depending on your date of birth in March given that today is the 15th, whether or not you have all the relevant documentation to substantiate claimed points and taking into account postage times from your current location to the UK you may or may not have a chance to make the 'under 28' category. If you think you will make the deadline submit as under 28 if not then as over 28.
2. Your intial HSMP Visa is valid for 12 months and you can enter the UK at any time during its validity. Depending on your level of economic activity in the UK during those 12 months you may or may not get an extension of your HSMP for another 3 years i.e. time on HSMP would be 4 years. People who delay their entry into the UK or are absent for a lenghty period in the initial 12 months have little if anychance of renewal.
3. Refer to the
HSMP guidance for relevant details on documentation. As a summary everything substantiating your points should be an original. You ask for advice on HSMP applications in your post - mine is that you read this document in detail to see excactly what is required.
4. Correct.
5. Current processing times are quoted as between 6-8 weeks. This would typically apply for a well documented application. Those where further enquiries have to be made or further requests for information from the applicant made appear to be in the 16+weeks time frame.
Good Luck