I did just fine doing a solo 3h SP murmur survival mission reaching lvl 750+ enemies, so safe to say it works lol
Why viral? I like big crits, and viral heat slash is an easy way to achieve this, the latter being already built into the weapon.
If you plan to outsource viral, you can go for gas if you own primed fever strike, however if you don't, blast can be more devastating due to its larger AoE.
The riven is purely for personal QoL/ enjoyment, [Quickening] is likely better.
Use Naramon focus school for free double combo gain, +50% crit chance and not having to worry about maintaining combo count.