There’s actually quite a lot to unpack here. So first things first, I haven’t tried it but that sounds like an autopilot bug not allowing you to maintain anything above 35,000 with alt selections. I’ll have a try later and see if I get the same issue. (Edit - I tried and had no issues capturing at 35,000, 40,000 or 45,000ft and holding there in ALT.)
Secondly, what weight are you at? the achievable FL will be dependent upon weight (and other factors too such as outside air temperature and CG). A certified ceiling is just that, an upper limit, not a guarantee that you can reach it under all conditions (nor indeed that you couldn’t technically exceed it in some conditions). For example if you don’t mind I’ll frame this post in reference to the A320, which is where the bulk of my experience lies, you would have 39800’ as the certification ceiling. Often the Rec Max (recommended maximum) level will be significantly lower until you’ve burnt off some fuel (or thrown some passengers out, but that’s generally frowned upon outside of parachuting flights...), or if the temperature is warmer than the ISA atmosphere you will also see reductions to the rec max.
Just in terms of terminology here to help understand you can think of Rec Max (which is just an airbus term), as sort of an “operational” ceiling which takes into account the actual configuration of the aircraft and environmental conditions and gives you 0.3g buffer margin, the ability to maintain level at cruise thrust and 300fpm climb at climb thrust.
That last parameter leads on to the next point, which is that the available excess thrust is tiny up there... if you think that going from max cruise thrust to max climb thrust can only net you a theoretical 300fpm up, you can perhaps infer that there also isn’t much thrust left to accelerate you either. So slow acceleration up near your operational ceiling would very much be expected behaviour! It is in fact a point that more pilots could do with appreciating, and has led to incidents in the past due to a lack of handling experience at super high altitudes... the drag curve takes no prisoners up there and you do not want to find yourself on the wrong side of it!
As for the speed, again I’ve not really looked much at the plane in question but bare in mind that above the transition level it is Mach number that will become your limit (Mmo) not IAS (Vmo)... that’s a whole other conversation though! The specs on the plane selection screen seem to suggest 451 KTAS as the max cruise Speed though? Where do you get the 440? (Edit - I have no issue reaching 451 TAS?)
not sure what you mean here?