If one is invited to someones house on shabbat and they are being yotzeh kiddush with someone they know mumbles the words ,but they can't make their own kiddush for whatever reason (only enough wine for one cup,doesn't want to embarrass his host) is there any heiterim which allow him to be Yotzeh kiddush,or would he have to somehow make kiddush on the challah without someone noticing? What should one do in this situation?
As a personal solution, to avoid such situations, when I am invited to someone's house, I casually mention before the host says kiddush, "Btw, I prefer making my own Kiddush." There are some Minhagim that each person makes his own kiddush (at least males above bar mitzvah). (not just on Passover). That generally resolves the problem of discomfort.
If they are using grape juice for kiddush, (or if they are using wine and have only enough for one cup; or it is expensive wine and you feel uncomfortable to ask for a whole new cup) then grape juice can be diluted up to 40% grape juice and 60% water (don't have source) and still require the Bracha "haGafen" which could mitigate that issue.
Perhaps these ideas would help without coming on to whispering kiddush to yourself before haMotzi.
Please note that our wine (as opposed to the wine mentioned in the Gemara) cannot be diluted, as it is already diluted. Any further dilution would leave one with a safek or definite Bracha of Shehakol. See comment below "To focus on..."
Another preemptive Eitzah(piece of advice) could be, if there is an Eiruv, bring a bottle of wine/grape juice for a gift to the host. This certainly alleviates the problem of not having enough wine. It also makes it easier to ask to say your own kiddush.