I have proof of development and marketing of concept, in advance of conflicting application that has been filed in advance of mine. Can I use this as prior art to insure that I can still produce my original product without limitation?
If this application was filed after March 16th 2013 then the AIA patent law applies.
That would be 37 USC 102
(a) NOVELTY; PRIOR ART.—A person shall be entitled to a patent unless—
(1) the claimed invention was patented, described in a printed publication, or in public use, on sale, or otherwise available to the public before the effective filing date of the claimed invention; or
(2) the claimed invention was described in a patent issued under section 151 , or in an application for patent published or deemed published under section 122(b) , in which the patent or application, as the case may be, names another inventor and was effectively filed before the effective filing date of the claimed invention.
As you can see, if the invention was not known to the public in any way, the fact that you knew about it does not help you (unless it was stolen from you). You do not get some kind of exclusion or license just for having invented it first.
However, if the application was filed pre AIA, the law was different.
Pre-AIA 35 U.S.C. 102 Conditions for patentability; novelty and loss of right to patent.
A person shall be entitled to a patent unless -
(a) the invention was known or used by others in this country, or patented or described in a printed publication in this or a foreign country, before the invention thereof by the applicant for a patent.
Pre AIA the US was a "first to invent" country, so you having invented it first could get you the patent granted and theirs rejected.
See https://www.uspto.gov/web/offices/pac/mpep/s2132.html for more information pre AIA.
I understand that your question is can you make your product without worrying about the other person's patent. If the public activity you have evidence of occurred more than a year before they filed, it could have been used as prior art against them. It is complicated but if you have a published document (their marketing literature), you could potentially use it against them in an Inter Partes Review.