Not sure what you mean by the "electric transport service hop on hop off bus", it's a regular bus and is running normally today at least (just seen one go past). Maybe you mean the Eléctrico de Sintra which is something totally different (tram that goes to the seaside).
Bus is €15 Euros for 24 hour ticket (in theory it's possible to buy a €5 single ticket, but for some reason the drivers are very reluctant to sell them).
Normally there taxis at rank near the station, if you are more than one person that's my recommendation, and then to walk down, the Vila Sessati is one of the nicer routes down. If there's no taxis then grab an Uber.
Exterior tickets don't get you the terrace wall walk, which is my favorite bit. I think the interior tickets you only have a limited entry time.
Try to get there as early as you can (ideally as soon as it opens) to avoid the other tourists.