Regarding your #1:
Deflection of a beam (channel or tube are hollow beams) is more a function of the Moment of Inertia (I) than the modulus of the material.
Example: an I-Beam . . . increase its stiffness by increasing its height rather than the web width.
Reason: I= cube(h) * b * 1/12
Another way to look at deflection is through bending equations like :
y = P * cube(L) / 48 EI
for a simply supported 3 point bend.
So, deflection is inversely proportional to I and E. To decrease y, increase I or E.
But you can't control E since steel is 30,000 ksi and most stainless steels are 33,000 ksi and aluminum is 3,000 ksi. You can always control the Moment of Inertia through part geometry.
Strength and striffness are not the same.
You rule out aluminum alloys that would require only a modification to part geometry or the addition of a supporting internal steel channel.
There are filled and reinforced polymer materials that are sometimes more cost effective to manufacture after capital amortization.
If shiny is what you want, why not chrome plate steel? There is also an acid chloride zinc plate that is just as shiny as chrome and adds the corrosion resistance value.
Regarding #2:
Ever think of placing a less frictional material between the rolling part and the tube? Acetal (tradename Delrin) is used a great deal to reduce wear on moving parts, which by the way is the definition of "bearing". You can machine a rod of acetal to have an ID equal to your rolling parts OD. 50 lb-f is not sufficient enough to cause compressive failure, as long as the acetal is thick enough . . . see Moment of Inertia above.
Regarding #3:
On what formula are you basing the need for a strength of 150,000 ksi??
Remember that strength based failures are one of only three types: tensile, compressive, or shear.
In bending a beam is placed in a state called flexure. This is a composition of tenile (along the bottom beam axis) and compression (along the top beam axis) and shear (on a continuous profile, usually at the center). Failure is usually shear.
Have you thought about conducting a shear and bending moment diagram on your part from end to end? You'll have to know the formulas for shear and bending moment. One does involve a differential.
Mohr's Circle is another way to determine max shear.
Find a Strength of Materials book or ask for assistance from other Mechanical Engineers.
