Because the stability and any steric effect of methyl versus butyl to the carbocation intermediate a likely small, then the ratios are probably very close to 50:50, though it could be 55:45 or something like that.
For ethynylcyclopentane, the stabilizing effect of a proton versus a cyclopentane are much larger. Does any of the minor isomer form? As Dan indicated, there could be less than 0.01%. I don't know if any of a prospective minor isomer does or does not form. In my opinion, people that are reasonably knowledgable could predict or expect that if a minor component should form, it should be very small. Therefore, the person asking the question should imply or in some way also acknowledge that the reaction either gives a single isomer or if a minor isomer were to form, it should be small. They may simply ask you to draw the major product or THE product. In that case, we should infer that a single isomer forms. If a minor isomer did form, we could infer from the solution manual probably means that it would be present in very small amounts. I doubt the person writing the solution manual could know that less than one in 6.02x10^23 molecules form on a molar scale. This is what you should know. If the solution manual says a single isomer forms, then the stability difference between a =CH(+) and =CC5H9(+) carbocation are so large a single isomer forms. A proton is much less stabilizing than a carbon.