The numbers for the thermal panels seem to be the same as for regular panels, meaning that the time-averaged production of a panel is 72% of its peak production. Thus you need about 41 mk3 panels per mk3 exchanger.
You are right that a mk3 exchanger can almost power two mk3 engines. Personally I prefer to buffer steam and allow for overdraw, so I use even more than 2 engines per exchanger.
Panels should conduct heat through all four of their ports.
As for heat storage, I'm still not entirely sure how Factorio's heat capacity number works, because the game uses the wrong unit for it. The mk1 molten salt accumulator has a heat capacity of 5 MJ from the source, but if that's 5 MJ per 1C then that's just ridiculous: one accumulator at 1000C would have enough energy to run a mk3 exchanger for almost 10 minutes by itself.
Edit: I just checked another thread on this discussion section and indeed this is how it works. So a mk1 accumulator powering mk3 exchangers holds a maximum of (1000-335)*5=3325 MJ of usable energy (below 335C you can't make steam with a mk3 exchanger). This is equivalent to 665 vanilla accumulators, while mk3 panels are equivalent to 3.33 vanilla panels, so the optimal ratio of mk1 accumulators to mk3 panels is 237.5 panels per accumulator. (I got 237.5 from 665*0.84/3.33.) 237.5 panels with 1 accumulator, 6 mk3 exchangers and 12 mk3 engines will give a steady 34.2 MW and peak 36 MW. So a reasonable layout is probably columns of 41 panels, with a row of accumulators and heat pipes at the top, with one accumulator per 4 or 5 exchangers.
Note that making way more accumulators than this will hurt you in the short term because the storage will take time to fill, during which you're not generating electricity. This is different from vanilla accumulators, which can happily discharge during the night even if they don't get fully charged up during the day.
