Re. Pit lanes - those diagrams are for previous Scalextric digital powerbases. The top and bottom configurations are not compatible with an unmodified ARC Pro powerbase.
The thing to always remember with ARC Pro is that the two lanes through the powerbase must be kept isolated - that means they can't be merged. That's why the old curved lane changers and the single-lane chicane (bottom diagram above) short out. Pit lanes must go out from the two lanes and then re-join. That's why the top diagram shorts.
Re. the curved lane changer mods - they are not 'easy'. I suggest you go back and follow the steps again for the mod that you decided to do. The two lanes must be isolated - so check all connections, cuts etc. If that still doesn't work, the best thing might be for you to post pictures of the top side and underside of your lane changer in a thread and we can have a look.
I hope that helps.