>Igreja de Santiago
Don't know! I haven't seen anybody who could give you a stamp (early morning Oct. 16, 2018). The church was open though.
>Cathedral
I had my credencial from there. It was was already stamped.
>Igreja de Nossa Senhora dos Navegantes
at the end of Lisbon (Parque das Nacoes), next to the motorway bridge. The short detour is marked. The church is open all day. Offices had been closed at midday in October, but were still open at 7pm (more than a year ago).
There is a bus stop nearby, so you can walk the town without backpack, get your stamp there and continue the day after. They also had some guide books to sell. The bus fare is still included in the 24h Lisbon ticket, I just don't remember the line number. Sacavem train station is about 1,5km.