Problems with rust on pipe

I have a back-up in my pipe leading from my main level BR. There is a trap in the basement in which I cleaned out but that didn't help. I can snake it until it reaches a elbow and thats where I have the problem. The elbow has a cap but it is rusted so bad I cannot get it free. I tried WD40 to loosen it up for the past couple of days off and on and it hasn't broken it's grip. (Yes, we have another BR - I am clean.) Anyway, I was told to take a lite torch to it to heat it up and then try - but before I do that I thought I'd ask around. I also thought I would just cut out that whole section, clean and re-attach, but again for now I'd like just to get in there and snake it out and this spring change out the whole section from BR to sewer. Thanks for any help!!

I think the quickest way is to just cut that pipe and do your cleaning and put it back with a no-hub band. Replace what ever in the spring..........