as far as his comment on air vs ground etc.
Legally, your supposed to use a ground freq with a ground vehicle, and air with air, airplane style controllers tuned to ground freq are a expensive and hard to find option, i dont waist the time when i need more than 3 channels on the ground.
dont worry about using a airplane controller, just make sure there is no RC airplanes within a couple of miles from you. usually that is not hard, most of the time you have to drive miles to even find someone using a RC airplane, just make sure no RC airplane fields or places people commenly fly are around you while using this,. get away from it at all possible so you can be *safe* for them and you while you do what you want. im going to get crap for saying that but if you want to go through the effort for a controller that you have one use for, and no one wants to buy when you decide to sell, thats up to you.
Im going to get crap for telling you this, but im just being logical, its all be looking out for your self and others, your probbly going to only use this at home, you have probbly never seen a RC airplane within miles, as long as that stays fact you well have no problems using a airplane controller.
Im not sure exactly what european style means, you may want to search that further, No you do not need 2 servers on one stick or one channel, you can have that if you want.
That servo *should* work fine, you want a high torq servo and that is one, im not sure about your mower they are all different, you may need to buy one and experiment with it, see how it pans out. when it comes to servos options are endless seriously.
also, airplane and heli controllers are basicly the same, the left side is a clicker, the right side is completely sprung, in more detail, the left stick is known as throttle/rudder, throttle being forward and back is on a clicker, it well sit where you leave it, the rudder fuction is taking that same left side stick, and moving it left to right.
one stick can control 2 servos indepentantly. the same for the stick on the right, except no clicker, one function is forward reverse, the other side to side, in other words 2 sticks control 4 channels completely on there own (Before you include buttons)
6-9 channels only difference is buttons, there is no radio that i know of with a 3rd stick honestly i think some people would be interested, i just dont know of it yet, at least not common.
as far as your servo returning to center, thats what they do normaly, when you turn on your servo you have a trim ajustment on the radio, center that and put your servo arm on centered where you want, you can trim from here, the servo well allways return to this netural position every time you let go of a sprung stick on the TX, your arm throw well have to be ajusted by the lenght of servo arm, or the position of the arm is controlling, in other words think leverage. when the servo/RX and TX are all off, the servo can be pushed around, when everything is on it holds a position that matches the stick that controls it.
that servo you linked to is complete overkill, you can get a high torq servo for about 60 bucks that well probbaly do the job. depends alot on how much throw you need and how much resistance it receives, you may want to do a little trail and error with this like buying maby a single servo. but the one you link to is a linear actuator, kind of like a hydraulic cylinder replacement, you can downgrade, but if you want to use that i KNOW it has enough distance and power to push! but you mentioned cost so =)
you also DONT need a digital servo, unless your flying high end Airplanes or driving a RC car in a serious race, but for your project style you just need analog.
the difference is a fine level of precision at increased price, you wont notice it at all with a project like yours.
i have a feeling this would still be overkill, but it would be far more cost effective than those big guys you linked to.
check the specs on that, 425 OZ of pushing power with a 1 inch arm, thats 25 pounds ish, if you make the arm 4 inches you cut that number in 4, its probbly still more than enough to push your levers around. then you just need to get a long servo arm off servo city, it would come with about a 1-2 inch arm at most infact call servo city and ask them about that, some of those monster servos may actually come with something decent now, i have had to buy aftermarket long servo arms for years for stuff.
i know the travel distance on those levers are probbly quite a bit, there is a whole equasion to figure out but im giving you the right idea.
as far as how compatible everything is, Your TX And RX need to be the same if FM or AM and also need to both be for 72 mhz only (72 is air) eather way that needs to match. basicly your looking for FM 72mhz TX and RX
Other than that, they pretty much work just fine, when you look up servo's they mention something like a Futuba J plug ( very popular) or a Airtronics style plug(not as popular)
those are the 2 majors, minor plug difference and the wires are in a different order if i rember, i would say stick with a futuba plug style setup, its extremly common and easy to use. As long as your servos are ment to plug into your RX, they well allways work. Servos are completely compatible as long as your plug type matches.
i think i covered your questions, if i forgot anything you have anything else to ask let me know.
edit: metal gears are cool, and now often cheap but i wouldnt consider them required for your project, they realy come in handy on 8 pound RC cars that get tossed around and slammed, the servo needs metal gears to combat the abusive forces and not break, for those servos pushing levers its not needed, but dont try to avoid it it probbly wont add hardly any cost, just a FYI that its not completely needed.