Yes Amb status gives you a 1-cat upgrade BUT if you are Amb and book 1-cat below club room category, whilst hotel will upgrade your room +1 category hotel, free lounge access is not an Ambassador benefit and is not given.
As Ambassador if you want Club Lounge access you need to book such a room category initially. You then get 1-cat upgrade ++ keep the booked lounge access
(Neither is award night upgrades part of ambassador benefits, but both bogo nights get full benefits.)

The Amb Bogo Cert gives you one weekend second night free, but at a small? cost as you are paying "full flex" instead of lower advance rate for the first night. Of course you may want to book flex rate anyway in which case no difference to your rates.

With IC Singapore you either book Friday and get Sturday free, or book Saturday for Sunday free. Check both Sat and Sunday Flex rates and book to get the most costly night free *if at different room rates)

Assuming buy amb status purely for this stay, and then you book IC Club Room as Ambassador and get usual 1-cat upgrade
Your Saving = 4nights rate diff of Club-Heritage upgrade + 1night full rate of 2nd night bogo upgraded Heritage - us$200 (cost of Amb membership)

Obviously if you stay other IC's later then you shoud not include cost of $200 in working out the saving

nb Lounge access at IC Sin gives you option of restaurant b/f foc