I haven't played this in forever but board game rules dictate that if you are facing the genestealer and roll higher you get the kill, otherwise you turn to face them. No overwatch with a flamer, assault cannon should have it unless they changed it. Stats, idk never got far enough for it to matter.

While SH Ascension allows you to build up close combat Marines, it is still playing your weakness to their strength. I prefer to invest in ranged combat - its how the Marines are structured to be best.

See here about melee mechanics: https://steamhost.cn/steamcommunity_com/app/317620/discussions/0/624075036322164840/?ctp=2#c624075036386793517

Terminators have one attack. Except with LC which give 2 total. Genestealers have 2 or 4 attacks.

Most, not all, lvl X Terminators want TS4. And some of them TS5. WS is of secondary importance. Most Sergeants with PS want to increase WS. Anyone designed for LC or TH/SS requires TS5 and WS4 minimum.
A DA ranged Terminator with AG5, BS5 and perception 5 is perfectly legitimate.

Overwatch is not linked to Terminator class. It is linked to the weapon. HF and PC cannot set overwatch. AC can. Which is a complete waste of ammo. So you probably don't want to. Cyclones come with a SB. Which can and should use overwatch.