2003 Dodge Neon Repairs, codes & costs

Twice within 18 months had similar problem. Both times car became very difficult to shift out of park into drive. Had lot of trouble going between 1st & 3rd gears & could not get up any speed. First time transmission fluid was low so more added & helped only for 5-10 min. Mechanic said the hose from transmission looping through radiator had leaked and filled radiator with transmission fluid, trashing the radiator. Radiator & hose replacement done, transmission fluid changed. Now about year & a half later it began doing the exact same thing. The first garage had closed, so took to a different mechanic who said he got a Code PO700 and it needed a new transmission. Did not go ahead with that, but took to a 2nd mechanic who diagnosed it with the same transmission fluid leak into radiator as the first time & again replaced radiator, hose & fluid. He did not mention the code (although we gave him the paperwork from the first place). Now the car runs fine except when slowing, it****** slightly going into lowest gear. Not constantly, but intermittently. 3rd mechanic says he pulled Code PO841LR and which is some type of sensor costing around $500 to replace. Are these problems related? Is the leak of tranny fluid into the radiator a common problem? Do we trust the last sensor diagnosis & fork over another $500, or should we be looking into this further? Is the $500 a good estimate for this sensor replacement?