violating a derabbanan to avoid a d'oraysa (II)

In the comments to the previous post, Bill Selliger suggested that the case in Shabbos is distinct from the Rambam’s case because in the Rambam’s case no aveira was yet done, while in the case of one who placed bread in the oven the ma’aseh issur is already done - enforcing the issur derabbanan of "rediyas hapas" to prevent removing the bread just forecloses the opportunity to undo the crime. I would agree, but have one hesitation: is placing the bread in the oven really a ma’aseh aveira? Yes, it inevitably leads to the bread baking and chilul shabbos (there is no ptur gerama here, see B”K 60), but couldn’t one argue that chilul shabbos is in the conclusion of the baking, not the act of placing dough in the oven (this is raised by achronim)? If so, until the baking occurs, why not undo the neutral act of having placed the bread in the oven?
The gemara in Shabbos (4a) adds a wrinkle to the equation and asks whether someone else can remove the bread dough in violation of the derabbanan of "rediyas hapas" so that our baker should be saved from an issur d’oraysa – do we allow “chatei b’shvil she’yizkeh chaveirecha”, doing a minor aveira to spare someone else a greater wrong? The gemara rules no. Tosfos quotes a contradictory gemara (Eiruvin 32) which says a cheveir may violate the “minor” prohibition of separating terumah “shelo min hamukaf”, without the bulk of tevel being present, so that someone else does not come to violate the more stringent issur of eating tevel. Asks Tosfos: isn’t this an example of “chetei b'shvil sh’yizkeh chaveirecha”, of committing a minor sin to save one’s friend from a greater wrong? The Riva”s answers in parallel to the approach suggested: in our case of the bread stuck to the oven, the baker committed the ma’aseh aveirah already and just needs a way out; by the tevel case, the person who stands to eat the tevel has not done anything wrong. Yet, Tosfos offers a second answer as well: by the case of tevel, it was the chaveir who offered his neighbor a basket of fruit and is the cause of the potential violation of achilas tevel - he must therefore find a way to undo the act and help his friend. By the case of the bread in the oven, the person who wants to remove the bread was not the cause of it being placed there. Are these simply two possible answers, or is Tosfos debating the issue of whether placing the dough alone is the ma’aseh aveira, or is there perhaps some other issue involved?
Tosfos further asks how one could be chayav for ofeh when there is always the possibility of “rediyas hapas”, removing the bread from the oven before it bakes. Riv”a answers that the ma’aseh aveira is putting the dough in the oven; rediyas hapas just undoes the aveira. Is this the Riv”a l’shitaso of the previous debate, or not?