ANSWER - I think you are better off following your instinct and celebrating when things feel meaningful to you. I see the Pagan wheel of the year as a guide rather than a structure of mandatory celebration dates and I see no reason why you cannot adopt the ideas behind the individual sabbats to fit with the seasons you are actually experiencing in your part of the world. Rather than trying to tie the sabbats to a date why not celebrate when you see physical evidence of the wheel turning? Imbolc can be tied in to the melting of the snow, Ostara to the birth of the spring lambs, Litha to the first heatwave, Mabon to the crop harvesting and so on and so forth.
The fact that weather dependent cultures will all observe these life events at different times is not really a problem - just acknowledge and honour the events when they happen in your part of the world. It is important to remember that when we celebrate a sabbat the focus of the celebration is not a date in the diary but an understanding of a new phase in the earth's circle. It makes more sense to me to celebrate something like a harvest when you can actually see evidence of harvesting going on around you and not six weeks before or after just because another country may be gathering their harvest or somebody has deemed it is traditional to celebrate on this date.
I do observe the wheel of the year but as I've commented before my participation in a sabbat can vary significantly depending on my mood and how engaged I am at the given time. I view nothing on my path as a must do - if I don't feel the joy of the harvest I'm certainly not going to spend my time celebrating it. I think sometimes this is the hardest thing for people with a traditional religious background to get their heads around - witchcraft (a secular path but for many closely connected with their religion and/or spirituality) isn't about rules and dates, it's about your own relationship with the world around you. Trying to force a craft path into a set of must dos kills the whole individuality of what it is to be a witch.
I've spoken on the wheel of the year before here with suggestions as to how sabbats can be celebrated when they differ to the commonly accepted dates.