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if a player gets a good result on attack test but the opponent successfully parry or avoids it can he still make a momentum attack?

Hmmmm.... that's a interesting question.

As a house ruling, I'd say yes.

Parry & Avoid Blow don't change the fact that you scored a Good Success vs the opponent's Phys Defense on the first Attack. Avoid Blow states that the incoming attack is dodged, and Parry states that the defending character deflects the blow and takes no damage. Giving the offensive character, who has Momentum attack, the extra chance at a Damage roll rewards that first Good Result test. That's appropriate for an advanced Journeyman Talent, IMO.

That's my read of it as well. The defensive talent (Avoid Blow/Parry) doesn't change the result of the original attack test, so I would allow Momentum Attack to be used.

Yeah.
Result Level is independent from success or not (which is one of the reasons it was renamed in ED Classic, it was previously success level. That way, you could have failed at something with a good success...)

Ignore the part where it talks about "after damage from first attack", that is only a reminder that you completely resolve the first attack before making the momentum attack.

If the blow was avoided I think I wouldn't allow. It's logical that you lose your balance (and momentum) when someone avoids your blow.

With parrying you can regain energy after weapons clash (3rd Newton law of motion: The mutual forces of action and reaction between two bodies are equal, opposite and collinear).
Of course after being parried you can also lose body balance, but it seems more logical to me.

I guess it come down to what you consider a parry. A really good parry utilises a weapon and body movement to redirect the arc/direction of the incommng blow enough that a commited attacker loses thier balance.

Then there is what i consider a block, simply stopping the incomming attack by meeting force with force, in EarthDawn i think the parry talent/skill is a bit of both.

In the SCA we were taught that a parry is a deflection leaving most of the speed in the opponents weapon and a block was absorbing the energy with our own weapon. In earth dawn the parry skill talent would defiantly include both, wich you use depends on your style and your weapon. It is very difficult to parry with a 2 handed axe, but you can block easy, if you block with a foil it will be broken in half and you will be hit, so you always parry.

As to using the energy of your swing to get an improved opening for your next strike its more about putting your opponent off balance than keeping energy in your swing. The need to parry (or block, or dodge) can put them off balance as easily as hitting them so it makes sense that it would always work the same.

Remember that it's not intended to stand for one specific thing. Neither does an attack roll.
Your offensive maneuvers could consist of three feints plus a light tap with the dull part of a sword plus a slash intended to be lethal, and it'd all be considered part of one attack, with one attack roll. Your defensive maneuvers for Parry can be performed with a weapon (and whatever weapon you use, would result in different maneuvers as explained by TZK above), or you could wave around your cloak (remember you can assign a DB for parry to _anything_ if you want), or use a shield (and parrying there can mean blocking it, deflecting it with the shield, or just bringing it into position to prompt your opponent to bypass it--where a successful Parry means the shield was put in the way, the opponent tried to get around, but that made the attack go into empty space) -- there doesn't need to be weapon on weapon or weapon on shield contact at all.
ED assumes you're doing the most efficient thing for the item you're using, and keeps things abstract so you can describe/imagine things the way you like them to be--and that doesn't necessarily impact mechanics.