I think I visited my shortlist schools three or four times between the open days, private tours (I did two for each), and stay-and-plays. Don’t feel awkward about visiting as much as you need to!
Often, your first visit just gives you a general feel for the layout and ethos, so you can create a shortlist. On subsequent private visits, I asked to see specific things such as classrooms versus grounds. This gave us a better sense of the classroom environment and behaviour, and also more time to look through the books at a slower pace. This was crucial, as the books at some of the TW preps were at a very different level and quite far behind the London school we were moving from, and we looked at both areas initially as we were also house-hunting.
I also asked to speak with the heads of pre-prep to discuss their curriculum choices in more detail, for example which maths programme they follow, why they use it, and how they differentiate for different abilities. On some visits, I requested tours that overlapped with playtime so I could see how the children interacted and treated one another. I still found open days helpful, as they give you the chance to speak with actual parents and teaching staff, rather than just admissions or marketing. Stay-and-plays were great too, as they allowed me to see how the staff interacted with my own child.
A friend who is starting at Somerhill next year is doing a parent and toddler or forest school session one afternoon a week, which might be worth investigating. Because we were moving from a London school, we also arranged for my eldest child to do taster days at both schools before finalising our decision. They liked both, the logistics simply worked better for one. Since we were moving from a small, very selective London prep, Hilden Grange felt like a more natural fit, although I was still torn as both were very good schools. I also think that if Tonbridge is your long-term goal, Hilden Grange is very hard to beat.
In terms of logistics, I would do trial runs at the main drop-off and pick-up times, park, get out, and actually walk to the school. It will either be perfectly doable and not as bad as you fear, or you will find it too much.
Good luck, both are fabulous schools, and it is really just a matter of finding the right fit for your child.