No, you dont have to reprogram the eprom if replacing the TP chip, but if you are only seeing 5V then probably, either the code is bad in the eprom (quite likely after updating) or the TP chip is bad, seeing you have replaced the TP its probably the code in the EPROM, if the TP chip doesnt call for 20V from the charger then the TP chip probably isnt starting due to EPROM code corruption, you will only see 5V no matter if you Sorin the mosfets or not,

Gate to ground should be 28V i believe, on my E15 the MOSFETS do not get hot with 28V to the gates, i have put a thermal pad on them...