words - Questions about 背負う

背負う is pronounced せおう, but many dictionaries also list a second pronunciation as しょう.

1. 
   

   I've never heard it pronounced as しょう in practice. In what situations does it have this pronunciation? Is that only an archaic pronunciation, or is it still in use? Is it a regional/dialectical thing? Is one free to pronounce it with either way, unrestricted?

   
   


2. 
   

   せおう (or おう, rather) conjugates as a 五段. Does しょう also conjugate as a 五段?

   
   

• ます形　→　しょいます


• ない形　→　しょわない


• ～て･た形　→　しょって・た


• 命令形　→　しょえ


• 仮定形　→　しょえば


• 可能形　→　しょえる


• 意志形　→　しょおう

Answer

The only difference I perceive is that しょう sounds slightly more "intimate" as if it's a regional dialect. But this doesn't mean せおう is less of everyday language. Probably I feel this way because the pronunciation is a naturally slurred version of せおう (i.e., /seo/ -> /syo/), which is most likely the result of a natural sound shift like 音便.

The tricky thing is that しょう here is not a homonym of しょう as in 賞, 章, and 小. While they look exactly the same in kana, しょう as 背負う does not have the double mora お. It's しょ as one mora plus う as another mora, i.e., it's not the same pronunciation as しょー.

As for its conjugation, it's 五段 as you guessed.