Your lawyer sounds too junior to be dealing with this!
What's the term of the head lease? If it expires in 2 years' time that's really different from one which has hundreds of years left on it.
Are the freehold and both leaseholds all for exactly the same land?
Not sure what your lawyer means when he/she says about future new charges. Landlords broadly can charge for 4 things:
If there is one, the amount and the timings/ amount of any increase is written in - the Ll can't unilaterally change it.
Insurance- if the freeholder insures then it will only be a technical pass-through of the cost via the F/H via the headlessee and back again.
Service charges. There probably aren't any services in practice but your solicitor should check how it works.
Fees for applications for consent eg for assignment (sale), request for any alterations which require consent. Again, your solicitor should be able to tell you what needs consent (possibly not much for a house) and how it works in practice.
It's absolutely the seller's issue to deal with and any indemnity insurance costs should be theirs.
Don't buy it and think that when you sell, you can just sell your leasehold if necessary as leasehold houses are not popular and especially not when's not something typical for the local area.