I agree with most of what you said except #5. How does a horizontal shaft motor do better on slopes and last longer than a vertical shaft motor? A liquid cooled vertical shaft motor with full pressure lubrication will last just as long as a horizontal shaft motor. Vertical shaft motors might have a bad rep since the low end lawn tractors all have vertical shaft engines and many are built really cheap and some don't even have oil filters. The high end lawn tractors and medium duty garden tractors (X300 and X500 series John Deere's) with vertical shaft engines are not cheaped out and built with the same quality as the horizontal shaft motors.
Horizontal shaft engines have different shape sump.
It is deeper and thus the engine can be tipped to a much greater angle before the oil pump is out of the oil supply.
Vertical shaft motors have a large shallow sump so quite small angles will have the pump pumping frotthy air not oil.
I keep an old sump which I fill with water then tilt to explain this to customers who come in with an oil starvation seizure.
My service footprint is very hilly and I get at least 3 seized engines every season.
This is the prime reason why the instruction book warns against mowing along a slope.
While a steepish slope can topple the mower a quite shallow slope can starve the engine of oil.
Second consideration is service life.
Both types have a design life of around 10 years.
However vertical shafts are only used on mowers and DOMESTIC mowers get used 1 day a week 20 to 30 weeks a year around 50 hours a year so have a 500 hour service life
Horizontal shaft engines go on pumps, compressors, skid steers, tow motors, trenchers, rotary hoes and the like which are designed to be used every day and their 10 year life goes 5000 hours.
Thus the horizontal motor is built to a much higher standard than a vertical engine, better quality materials and better made which is reflected in the price.
Two otherwise identical looking engines, the vertical shaft will be nearly 1/2 the price of the horizontal engine because it is made cheaper to be used almost excluseivly in the very price sensitive lawn mower market.
Honda was the big exception, their engines were almost the same which made them too expensive for use in lawnmowers because not even commercial users were willing to pay the extra for a superior engine that would pay for the excess price tag within the first 12 months. Honda refused to downgrade their engines so they exited the vertical engine market.
Not too long ago. all the top end commercial mowers used horizontal shaft engines and lot of the better ones still do because the engines are better.
There is a world of difference between a forged steel crankshaft and a cast malleable iron crankshaft.
The former is found in nearly all horizontal engines and the latter is found in most vertical shaft engines and all domestic vertical engines.