Hopefully i can help..
First checks to make would be the speedo sensor/plug connection contacts and sensor wiring by tracing back for breaks etc.they maybe able to check for 12v on one wire,an earth and the signal wire to check continuity between the speed sensor end and the dashboard/instrument..quite often the sensor itself can be at fualt and is located into the diff casing of the gearbox,sensor can be aquired from Renault...However on some without the speedo sensor on the gearbox then yours if fitted with ABS your speedo/odometer will take readings from the ABS system via the wheel speed sensor and sometimes if thats the case then an ABS warning may flag up.so these would be other areas to have checked before the dash cluster is replaced if need be,some online companys can repair clusters so it maybe worth enquiring with them too once the cluster itself has been confirmed faulty http://clusterrepairsuk.co.uk/ ..if not then it will have to be a new one fitted...
The odometer will be in conjunction with the speedo or take its readings from that via the ECU..if the sensor wether it be on the gearbox or if yours is part of your ABS system..then if any of those fail or the signal is interupted in anyway caused by a wiring/plug fault or even the sensor itself then the odometer can't see wether the cars moving hence the same situation with the speedo if you see where i'm coming from.....
The sensor on the gearbox should look like this as on ebay for example..
with the first question it would depend exactly what they found "if" the problem you had flagged up any engine management lights / fault code(s) and what checks they made according to which faults codes where stored before they concluded and roughly pin pointed the fualt before replaced anything...and very often it can be easy to suggest wiha problem like you have as simple as sparkplugs/leads/coils, though ecu can't detect wether sparkplugs are faulty / niether the leads / intake system leaks after MAF,vaccum system leaks / bad fuel or pressure,or delivery so no fualt lights..but only in a sense the coil(s) if faulty since they are controlled via the flywheel sensor/ecu...the engine itself is similar as in the Clio car..which does have well known issues regarding the throttle units causing power loss issues / Poor idling / engine missing / rough running / intermittent cutting out.so it could be as you have mentioned in your second question quite more than likly the throttle body was the issue, again usually this should flag up specific fault codes i would of thought depending on the machines capability..The electronic potentiometer/angle sensor will be inside the throttle body electronic side inside the black cover which your multiplug is connected too.the black cover is usually rivited so access will be restricted so it would be hard to se it unless you have a spare you can strip down..there will also usually be a position sensor at the throttle pedal which you will see a multiplug connection going to .which the ecu monitors and controls position the throttle flap as per the position of the pedal.