Yea It's an added advantage. You have extra points fpr that. IIM's do that to increase batch diversity. Freshers are also equally preferred as the people with work ex. If at all, generally a work ex of 1-2 years is good to pursue an mba.

Commerce have slight greater weightage, but they have a greater a plus point after getting into iim. Some subjects are related to commerce. Work experience is not mandatory, but it is good if you have some. I would suggest having 24 months of experience

Yes, non-engineers earn extra points for Academic diversity which is one of the various factors used to calculate your comprehensive score for making the merit list. The weightage of the said component is different for each IIMs. So yes, non-engineers have this advantage over Engineers.
Regarding your query on Work-ex, again each IIM has a scale to award scores for Work-ex based on the time period. Freshers off course get a 0 and a high workex is also valued less for some of the IIMs. The ideal Work-ex is of 2 years.
But having said that, a candidate will not be selected just because he/she is a commerce graduate and has a decent Work-ex. A lot depends on other factors as well especially WAT and PI.

For your preparation being from a commerce background can help you as it forms a base as the syllabus is quite similar but not while selection. Incase you think the commerce students have an upper hand in the selection process then this is not true.

Incase you have a work experience of 2 years and above then it shall be considered. Also it shall be seen how much relevant it is and what have you leant from it or what skills you have gained from it.
Work experience is not mandatory. If a student is a fresher then also he/she is eligible for admissions in IIMs and keeps good chances if the percentile and other parameters are good than  the one having experience.

To get into IIMs main considerations are:
1. Past academic scores
2. CAT percentile
3. Work experience
4. Other selection rounds

You can check the admission process for top 3 IIM's below:

IIM A

IIM B

IIM C

All the best.

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