Dear Peter:
The grave is correctly marked as Tec 5 . Corporal WOULD NOT be on the stone. It was the "equivalent to", but a serviceman would be referred to and called by, Tec 5.
The special skills could be a range of things. In my dad's case he was trained mechanic and placed with motor pool in Headquarters and Service Co. He not only worked on the WWII vehicles, but could run the machinery to make the repairs and many times had to fabricate parts to keep things running.
Yes, the combat engineers were armed, and many times fought as infantry. For the most part they were always at the FRONT LINES in harm's way. They had one hell of a job. Just ask Rocky, he will attest to that!
The combat engineers in the ETO used the standard issued radios issued to the Army, amongst them the SCR-536, Walkie-Talkie.
My buddy Carl Furtado is a 36th Combat Engineer and worked as an radio operator. You can read his diaries here, if you are interested:
http://www.6thcorpscombatengineers.com/CarlFurtado.htm
Proud Daughter of Walter (Monday) Poniedzialek
540th Engineer Combat Regiment, 2833rd Bn, H&S Co, 4th Platoon
There's "No Bridge Too Far"