Fuel cross feed after fuel leak.
 

I'm not familiar with the E190. However, in general, once QRH procedures are completed, you should manage the failed system and the airplane to the best of your knowledge and capability. If there is no QRH prohibition on using crossfeed, and you are confident you have isolated the leak and can proceed without re-introducing it, I would go with preserving 2-engine operation.

Not really enough information to answer specifically, and I definitely am not familiar with the E-190 or the QRH procedure, but here are some of my general thoughts.

First, did the QRH procedure actually isolate the fuel leak? If not, then reinstating the cross feed may (literally) piss away additional fuel. This might actually be okay if you have more than enough fuel to get to your point of landing, but may leave you short otherwise (for instance, Air Transit Flight 236, August 2001). Second, if you are actively leaking fuel as you land, it may create an additional hazard particularly if the leak is in close proximity to a running engine. Even if you shut the engine down right after touchdown, it will still be hot and could potentially ignite any fuel vapors in the area. Finally, there is the issue of lateral imbalance. Depending on the exact scenario, cross feeding may hurt or help maintain the imbalance within limits.

Unrelated cause, but, imagine how much better the ATR accident in Taiwan might have turned out if they hadn't hurried to shut down the engine.

I don't know the EMB-190 QRH, but I was involved in the evolution of the Boeing fuel leak procedures over the last 30 years, and the system architecture is effectively the same with respect to this issue. The reason they want you to stop any crossfeeding in progress is to allow you to determine whether the leak is in the left or right engine fuel system. (Large leaks are almost always from the plumbing, not the tank itself.) Once you have verified a leak on the left or right side by comparing fuel flows and watching FQIS dropping over a period of time, the procedure (at least on the Boeing airplanes) is to shut down the engine on the affected side to stop the fuel loss primarily to preserve range, and also to reduce the risk of fire. You then crossfeed as needed the same way you would for any other engine shutdown. Depending on the mission, the fuel leak rate, and where you are in the mission, you may need all of the fuel in both mains to make your destination, alternate, or diversion airport on your remaining engine.

So, Nomad2, you were correct that you don't just keep the crossfeed closed, but the second instructor probably expected you to shut down the engine on the affected side, which I suspect is the QRH procedure. Keeping the leaking engine going and balancing into the leak is what led to the A330 dead-sticking into the Azores. This link tells that story if you aren't aware of the event.

https://lessonslearned.faa.gov/ll_ma...=73&LLTypeID=2

I can talk about Airbus. After the fuel leak procedure has been run and the fuel leak has been stopped only then cross feed can be considered. This can only happen if the leak was from one of the engines and the procedure would have asked the engine be shut. Now if you need the fuel from dead side to reach destination you can use the cross feed. Otherwise for handling purposes balancing is not required. If the leak was from the wing tanks or center tank then the engine is to be kept running till it flames out or aircraft is landed but cross feed is to be kept shut.

Nomad2
You may not have considered whether the leak was identified from the engine or the tank. Both checkers could be right. In the first case the leak may have been from the tank so you decided correctly to keep it on and proceed to diversion land on two engines or if it flames out then on one. In second case the leak was from the engine and it should have been shut. Keeping it on in this case was incorrect. Also after confirming leak has stopped cross feed should have been used to give you access to dead engine side fuel.

In some scenarios, you need to preserve all the fuel you can get. Even if it means shutting one, keeping the cross-feed open and transferring the content of the leaking tank to the opposite engine while you still can.
In some scenarios, you may wish to keep both engines running. Even if it means with one wing already empty, transferring to that engine from the opposite good side at the expense of letting the oversupply drain out of the defective tank.

If the procedures for one of the above would be mis-applied, (that's what human elements are for), the outcome might be fatal.

Without knowledge of the E190 (myself on an ancient type), I think there will be only one fuel leak NNC. That NNC, as written, will typically need to provide one-solution-safe-for-all drill (typically fuel preservation)

After you have applied that NNC there might be room to apply core system knowledge and sound judgement. But that is very different from the goal of the checklist's designers and people who approve such. One also might make a very big mess of it.

Nomad2 I think the relevance to your second experience is not with Air Transat (they did not understand the problem until too late), more with Air Asia (unauthorised reset).

See, SIM sessions have a plan and the good ones have a training objective behind them. To execute a correct NNC properly and stick with it is a desirable skill, maybe that is what the instructor was authorised to train you that day. Perhaps he needed you on one engine for the remainder of the exercises (sure, there are buttons for that at his station).

In FUEL LEAK procedure the first action after leak confirmed is:
LAND ASAP in red. So considering leaking tank quantity as not available a suitable airport should be chosen as diversion and aircraft headed towards that. Everything else comes after that. Whether on one engine or two engines will be guided by the procedure.

The purpose doesn't include fuel leak. Also simply switching on Xfeed doesn't do anything unless you switch off fuel pumps from one side. If you did that on the leaking side then you will only end up loosing fuel from non leaking tank. And you did it on the non leaking side the leaking side engine will flame out early. Just follow the check list. Don't device a new one.