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Lawnmower blades are very forgiving and any sharpening will be infinitely better than how they come from the factory. I have typically used an angle grinder in the past. However a flat single cut file will produce as fine an edge as you will ever need. I would not even bother with the tormek to be honest.

I'm not sure I understand what you mean. But do you mean that aluminum oxide won't cut mentioned steels?

Yes and no. It is a matter of hardness. More specifically MOHS hardness. There are different forms of harness but it is MOHS hardness which determines the abrasive for each material. In the MOHS hardness cscale each material is given a number form 0 to 10, 10 being diamond and 0 being talc. For an abrasive to be able to sharpen it has to have a higher MOHS hardness that the material being sharpened.

For steels it depends on the composition and the temper. Different steels of ther same composition will have different harnesses depending on the temper. Knives often consist of several different steels so you may easily be able to sharpen the casing but have difficulty with the core.

Aluminium oxide has a range of MOHS hardness depending on the impurities but in general it can harden carbon steels but not so much high speed steel. So it will vary. When I was sharpening a very hard Japanese knife, my HSS thicknesser blades or my HSS drill bits the SH wheel glazed rapidly. It did cut the steel but was very slow and the wheel had to be constantly graded. It proved to be extremely inefficient. The issue was that the steel had a higher MOHS hardness than those steels. It may have been that the steel fell into the range of the softer to harder particles within the SG wheel this glazed rapidly.

The glazing happens when the steel being sharpened breaks the grit down since it is harder than the grit and you end up with a smooth wheel unable to cut the steel. This is an indication that the wheel is too soft.

Bottom line is that for efficient sharpening you need a harder grit that the material being sharpened.

I may try the existing stone in the meantime just to see what happens

You can use the SG so long as the jigs et all allow you to use it. The only difference is that the hollow is slightly deeper due to the reduced radius.

Many times DE-250 (grit 600) is the final step

The DE is 1000 grit.

Is there any practical or theoretical pros with using diamond (or CBN) as the last step of sharpening super steel's like m390, rather than traditional aluminium oxide stone?

Your abrasive needs to be harder (MOHS) than your steel or you will simply glaze your wheel. This is the process whereby the sharpening process rounds over your grit and leaves it blunt, thus making the wheel less effective and deforming rather than scratching the steel. I am assuming that you are dressing the SG otherwise you are leaving it glazed and not working efficiently.

The composite wheel is a lower grit and will not leave as fine an edge as the leather wheel. On the other hand honing on the composite wheel will be faster that on the leather wheel. As for customer experience it all depends as they say. The composite wheel will leave a toothier edge and for many operations will cut more easily as a result. For very fine and precise "food craft" even the leather wheel will leave too coarse an edge.

For general sharpening your average kitchen knife the composite wheel will be fine.

new drive wheel seems of much higher quality than the old one

Yes. They upgraded it significantly. The old plastic wheel would crack over the years.

if I let go then reapply pressure to the holding points on the wheel.

I hold the three arms to the wheel as I adjust. I think that is how you are supposed to use it.

during shipment the machine was damaged so the the motor is misaligned

Just what I was about to suggest. Definitely not right!!!

Agree completely with the previous answers. Just a couple of additional thoughts. Tormek say that the SG when fine graded gives you 1,000 grit. If you want to then you could then hand sharpen on the 3,000 stone then strop as you normally do since you would not be changing the bevel. What I do on high end knives is to strop on the Tormek wheel then hand strop with a quality green compound. On my nice chisels and plane irons is to do a few strokes on my 5000 grit ceramic stone and then hand strop. It is maybe 30 sec extra but seems to give me a slightly better edge. I also often refresh the edge on my woodworking tools on the 5000/hand strop which keeps it fresh and go back to the Tormek for major sharpenings. 

I'm looking for some cheap tools to practice on

If you do a search on AliExpress for "wood turning tools" then you will see quite a selection for some very reasonable prices. Make sure yu select the appropriate tools for the type of turning you want to do.

Yes, it does sound like something is scraping. The shaft may be slightly bent. Check the body and wheel to see if you can see any wear marks where it could be scraping. Not sure that it will really do any damage but do check with Tormek.

This could work - https://forum.tormek.com/index.php?topic=5335.0

Hmmm, not sure that it is vitrified but I cannot see that it would damage the bond in any case.

A few thoughts.

Is the wheel on the machine the SG or the SJ? It is not clear from the photos. If it is the SJ then you will take forever to get a bevel. If it is the SG then do you have the grading stone to change the grit? You reshape the bevel with the coarse grade then finish with the fine grit and then if desired the SJ wheel.

If you ware having trouble holding the jig down then toy are using too much pressure on the knife. Let the wheel do the work. If so then it may be you are not regrading the SG to the coarse setting, see prev para.

My old T7 was a bit noisy, did not worry me too much. Also if the honing wheel has a bump then it is perfectly fine. The leather is not always perfectly smooth. That will not impact the honing.

Did you clean and grease the shaft? The recommended is white lithium. It is water resistant and really sticky. Remove the shaft, clean it with a rag and re-grease the bearings. It pays to repeat this every few months, especially if you are using it a lot.

As for the heat it can get quite hot without doing any damage but if the shaft is not properly lubricated and/or you are using too much pressure then it can put excessive load on the motor.

My first guess would be to use white lithium grease

This is what I would use. Very sticky and would not need reapplying all the time as much as a lighter product.