It sounds as though you are confusing the owner of the note with the servicer--Fannie Mae (Federal National Mortgage Association) is likely the owner of your note, and Seterus is the current servicer. Noteholders regularly change servicers, which is why you have seen so many different entities involved. MERS is generally just a nominee, which is a whole other kind of entity. As far as who can foreclose on the property, under Florida law the owner or holder of the note and mortgage can foreclose, or their authorized representative can do so.
You are correct that this is all very confusing and is a kind of "shell game" that the banks play. Your best bet is to find a qualified foreclosure defense attorney in your area to help you make sense out of it all.
The mortgage industry and its foreclosure firms play neverending games of "hide the ball". Your recital of facts is a classic example. Odds are that Chase was the servicer for Fannie Mae, and that IBM - Seterus is now the servicer for Fannie Mae. Fannie Mae is like a big spider hiding back in its web, sending other parties out to do its dirty work.
The bottom line is that you need an experienced financial services litigation attorney to take this on and to push back in the ways that make sense in your case, with your particular facts.
Please note that the above is not intended as legal advice, it is for educational purposes only. No attorney-client relationship is created or is intended to be created hereby. You should contact a local attorney to discuss and to obtain legal advice.
Assuming that Seterus is the loan servicer for your loan and they have authorization from FNMA to bring the foreclosure suit, then yes Seterus can foreclose on your property. Seterus currently only services FNMA loans and this is a fairly common situation. However, based on the prior transfers you described, it is possible that you still have a "lack of standing" defense. It would be necessary to review the complaint when it is filed, the original note and mortgage, and FNMA's records regarding your loan. The endorsements on the note will be important and should be compared with FNMA's loan history. If FNMA does not have the original note and mortgage, it is more likely that there is a standing issue. This information can be obtained from the documents filed in your case and through discovery conducted during the suit.