hi, i like you draw at 1:1 (as most do) and also like you use paper space to compile all my drawings on the layout tabs.
scaling the v-ports for the different details.
I then add the dimensions and any text on the paper space layouts.
as you enter a dimension you use the snap function to establish the correct dim. this is in paper space and the v-port also in paper space.
you can if you wish change the v-port to model space whilst in paper space and enter a dimesion back on the original model. not quite sure why you would want to do this, you may as well stay in model space and complete your drawing there. but of course one would have to scale up the dims.
as regards not being able to snap whilst in paper space this happens to me alot,
I go to the layer tab at the top and reset(hide or unhide) the v-port layer and it seems to cure it.
another thing to watch out for is sometimes (for me) the v-port view flips from p-space to m-space whilst clicking through from model space to paper space layout to layout and this also sometimes stops the snap command wilst trying to add dims. therefore just keep an eye on the bot right where it indicates the current v-port, also you need to lock your v-ports after deciding on a scale.