How to best clean unsealed kitchen grout so that it can be sealed and/or regrouting later
I have done this many times. Yes, I am an electrician, but I grew up in a family business doing everything and working for a electrical contractor that paid me more as an apprentice. 

If the grout is solid (it looks good) I would use a very weak solution of muriatic acid and water, maybe a cup of MA to a gallon of water to clean it out. After that, triple rinse with with clear water, and after it has dried it can be sealed. 

I do see a few minor voids but the weak acid solution will clean out the area. It may take a wet vac to pull the moisture between rinses but once done it will be clean and a fresh surface to seal. If it looks uneven a 2nd treatment with maybe a stronger muriatic acid/water may be needed. 

I had to do this on my mom's new countertops a while after dad passed many years ago. I have resealed the counter tops several times over the years and they still look great after more than 35 years. Please remember always add acid to water (to prevent splashing of full-strength acid) and use gloves. Muriatic can be purchased at many big box stores and pool supply outlets.