Hi George!
You have asked quite o lot of questions so I will attempt to answer them one at a time.
The Meade 12" Light Bridge is a very good scope. It is a truss tube version of the standard GSO 12" dob that GSO make for Meade. The optics in these are mass produced so can vary a little in quality so there is the occasional poorer quality mirror but all the ones I have seen have had pretty good mirrors. Mechanics are OK and overall they represent very good value for money.
As to whether there is something better to invest in, well yes, but like all things telescopic it a matter of finding the best fit for you and your circumstances/budget. Moderate sized dobs like this give a large aperture at a very reasonable price. There are lots of scope that are better optically but cost many time more or have much smaller aperture. Just also be aware that they tend to be bulky, despite the truss tube construction.
They are not really a planetary scope, they give pretty nice images of planets but I found with my 12" that it struggled if I pushed the magnification beyond about 115x so planets were still rather small.
Your query about the BK7 versus Pyrex is one that crops up occasionally. I believe that Pyrex cools down slightly faster than BK7 but the main issue with BK7 versus Pyrex is that BK7 has a higher Coefficient of Thermal Expansion than Pyrex. That means that it expands and shrinks more than Pyrex as temperature changes. The effect on the figure of the mirror is potentially detectable but at f5 it would be very minor. A faster mirror would show it more. Personally, I wouldn't worry about the mirror material.
As regards the 12" versus 16" query. A 16 will show you more, more deep space objects are within reach, you can use more power on the planets people start queuing up at your scope at Star Parties!! BUT, besides the cost it is a bigger scope. I would always advise someone to at least have a look at one of these scopes before committing to buy (and that also applies to the 12") and make sure they are happy with the physical commitment and storage space required.
As to your friends query. A 10" newtonian ( remember that a dob is a type of newt) will usually be ok up to about 100 to 150x power so the 10mm EP he already has will give a pretty good idea of its capability as that gives 125x. At that power planets are still going to be small. Pushing the power also magnifies any atmospheric problems so one you push into higher power you are really waiting for those fleeting patches of still air to enable clear views. He could go out and invest in a better EP, but no EP will compensate for the atmosphere.
Malcolm