If you are in civilian law enforcement and in the military reserves, can you volunteer for active military duty or is that forbidden?

Nice scenario, but the opportunity to volunteer for repeated active duty activation's probably won't happen very often. Besides that, pressure from his civilian LEO command structure would probably be brought to bear.

First of all I think you need to familiarize yourself about reserve/guard military retirement pensions and how they work. They are not just based on you having 20 good years of military service. You must have 20 good years with the minimum number of points towards retirement each year for all those years to count. Now that is not hard because your 2 week annual tour together with all your weekend drills (UTAs) for each calendar year will give you a good year towards retirement and your pension would start at age 65. A deployment or any other active duty time just gives you more points towards retirement and that would then increase your pension slightly but not decrease the age at which you could collect it. That age at which you can collect a reserve or guard pension might be 60 if you had any time in the military prior to 1992. Otherwise they changed the age at which it would start to 65 for anyone serving after 1992 in the military for a guard or reserve pension.

As for you volunteering verses being activated involuntarily I would be careful with respect to that. While you have the letter of the law behind you in they taking you back once you come off active duty you volunteering to be activated and deployed for 8 months to a year might not sit well with your department. You would not be the first guardsman or reservist to then be at odds with their civilian employer because of your participating in the guard or reserves and especially if you were to voluntarily be activated. So I would tread carefully with regards to that.

For you to collect your pension any earlier than 65 you would have to qualify for an active duty retirement and have a lot more points than what you could accumulate probably. Points do increase the amount of money though that you will be paid in with that reserve/guard pension and there is also a max number. I maxed the number of points out that would count and fell just short of qualifying for an active duty retirement and you have to be on active duty for years in order to qualify for an active duty retirement. Your career counseling and pay people can answer all your questions about this and its important for you to understand how this all works though.

no it isn't, but keep in mind that some civilian jobs serve the public sector and are vital. Law enforcement is one of them.

also it is one year continuous, not one year cumulative. furthermore, the chances of being approved to go Active duty is pretty slim anyway.

In general, law enforcement agencies are very supportive of reserve and guard. There are many people who do both.