other than the avenger, best gimped character to use?

Asari duelist. You don't need lance or backlash at all.

I skipped out on Nova and just use it for the invulnerability frames, speced Fortify for more melee dmg and priming.  Works well on gold as long as you pay attention to what your doing and don't charge into a spot where enemies are around the corner from your charge target waiting to melt you.

Krogan gladiator actually works great at level 20 rank 1 since pull and nova aren't great skills anyway allthough I guess maxed pull would be kinda nice. Same with asari adept melee build since lance and throw are both quite shit. Same can be said for the duelist allthough I think that class is pretty average. Huntress is another one, again throw and lance are pretty crap especially on her so just max passives and stealth grid.

Angara insurgent works fine at a low level, just skip offensive tech (and ignore turret alltogether) untill you rank him up more, he'll still be really good with a lot of tankiness from his bioelectric defence/shield boost and good damage + cc with stun tripmines.

In a way I'd say avenger is one of the less good characters to have at level 1, yes it's still a really good class but she actualy has a lot of great skills you'd want to put points into. Unlike the huntress where I'd be completely fine without having points to put in throw or lance since even maxed out they won't make much of a difference. But being able to put 6 ranks in stickies or stealth grid on avenger does make a difference.

Probably not, Multiple enemies are stil going to melt you.  in the later rounds you going to need to deal with shielded enemys which will slow down to time to kill majorly, you run into one of them and then even another lowest lvl mook or 2 and you going to run into trouble.  You have to try and stick together so you can kill as fast as possible and pay attention to the mooks around you and get out when you see a lot in your LoS.  Fully speced out Kroguard only has like 207 shield or something they don't last very long at all and then they have a lil over 700 health (regenable when speced) seems kinda low really my human inf has more health and shields.

Fortify at rank 1 gives 45% damage reduction - that's all you need so max nova instead.

Why would you max out flamethrower? I have played the Merc extensively and its best use is for CC via panicking. Flak cannon actually has more damage/utility. I would argue with the Merc that maxing out Rage and Passives is far more important EVEN if you go flamethrower.