The Shulchan Aruch OC 268:3 paskens that if he says the words, ‘atah chonen’ out of habit, he must complete the bracha of ‘atah chonen’. The Biur Halacha questions this ruling, arguing that just because he said the word chonen does not necessarily make it that he prayed a weekday Shemoneh Esrei (and is no different then one who starts ata by Shacharis). He ends with a sarich iyun.
The Aruch HaShulchan definitively rules that even if one said ‘chonen’ or other words of the bracha of ‘atah chonen’ out of habit, then he does not complete the bracha, rather he returns to the Shabbos Shemoneh Esrei.
Should one be noheg like the Aruch Hashulchan as we say safek brachos Lehakel (especially as the Biur halacha is unsure)?
There are indeed poskim that agree with the Aruch Hashulchan, see Kovetz Halachos Shabbos 1 chap. 8-14, ftnt. 24. On the other hand the Mishna Berura, although he questions this, he paskens in s:k 7 that one should continue saying the rest of the bracha. This also seems to be the understanding in the Shulchan Aruch 268-3, which is based on Rishonim, and none of the major commentaries on the Shulchan Aruch argue with it. The rationale for this is that essentially, we could daven the middle brachos on Shabbos, and chazal omitted it because it is long, however if he started saying it already there is nothing wrong with continuing it.
It should be noted that a number of poskim (Eshel Avrohom, brought in Orchos Chaim -4, and Minchas Shabbos 77-17, Ketzos Hashulchan 78—ftnt. 2) say that if the person didn’t say the whole word chonen, rather “cho”, or “chon”, that it isn’t considered starting the next bracha, since he didn’t say the whole word, and he can continue with the Shabbos davening. We do see from this that all of these poskim did not say like the Aruch Hashulchan. Therefore, it seems that if one said the word chonen, that he should continue the bracha, and then say the Shabbos davening, which is the simple understanding of the Shulchan Aruch would seem to be correct. Also see Piskei Teshuvos 268-3.