Planning ahead and handing in the application as soon as possible is always a good idea since unforeseen problems, such as incomplete documentation, may arise and prolong the case processing time. However, a visa application cannot be accepted if handed in more than 6 months before the intended departure.
Do I need to submit the original invitation letter?
Yes. The reference - in business cases - must use the online invitation form. The invitation code, which you will receive after applying online, should be submitted to the Embassy . Information about online invitation letters https://www.nyidanmark.dk/en-us/forms/guide_to_online_application.htm
Can I apply for a visa with a validity longer than six months?
Tick 'Multiple Entries' in field 24 of the application form and write underneath that you wish for a long validity visa e.g. "I would like to have a 1 year visa.". You can also write your request for a long validity visa in a cover letter that you submit with your application.
Whether a visa for longer term can be issued depends on an assessment of the application.