Honestly you should call them by what works for the both of you.
I have an extended family as far as in-laws as a result of divorce. So my husband's mom had re-married and his dad as well. So typically for the "step" in-laws I call them by their first name (kind of a cue of what my husband calls them) - my kids still refer to them as grandpa, grandmom (maybe with their first name after it if they are around multiple grandparents) - this seems to work.
Unfortunately my husband's dad passed on right before our marriage so I have not had the pleasure to refer to him directly by any name.
For his mom - it kind of came about organically. I was writing a Christmas card for them and I addressed it as "Mom/Grandmom and R-/Grandpa" - she happily responded to my husband that I addressed her as mom (even though this was signed by all of us) - she probably realized I filled out the card and it was me that wrote a few things. This seemed to make her very happy so I call her mom.
I do have my mom around and I don't think she takes this personally and she certainly is my mom! I figure there is enough love to go around and no one feels petty or upset that you could also love another mom.
I think you two as a couple make mountains out of molehills. Really does it matter what you call someone (on either side of things you or your wife). As long as the parties (the addressing the person and the person receiving this reference) are comfortable with it - would the h*ll really cares?
Just be happy that they are around so you can call them something.