It is a hassle, but possible to manipulate an erection without the internal device. I opted not to get the device (despite the wonderful benefits there would be!). A condom wouldn't add enough firmness (...trust me :p ). You can buy external "Sleeves" from sex toy shops (online or in person).
I would say youre right that different procedures can leave you with varying levels of firmness of the penis, I have heard that the forearm phallo is probably the most flaccid/soft, so would require more "firming up" for sex!
So it is possible to have sex without the device, but unfortunately, it is somewhat of a hassle/pain to fiddle around and sort it, especially in the moment!
Also, I didn't have surgery in Thailand, so, sorry that I cant help there.