Urgent Help Needed with Title Issue

I'm at the point of exchange of contracts on sale of my house after 5 months. The buyer is now refusing to proceed for the following reason:

My house was shared ownership/leasehold when I initially bought 50% through a solicitor. I stair cased up to 100% and then registered my ownership of the freehold at the Land Registry myself without using a solicitor (I know now that this was stupid and has caused this but there is no point going over it now). I did not realise until now that there were both leasehold and freehold titles still in existence on the property side by side; they should have merged and become one freehold after I purchased but this didn't happen. 
My solicitor reassured me that this is irrelevant; that the house is in fact 100% freehold and since I am now effectively my own landlord the leasehold is to all intents and purposes defunct. They told me initially that the buyers solicitor would apply to have a merge done on completion and I had thought there wouldn't be much of an issue.
The thing I do not understand is they also said they were "almost certain" but "could not guarantee" the merge would be approved after completion. This is what has put my buyer off.
They also said if I applied for a merge myself it may not be approved at all (as there is no "intent"?).
The buyer will now not sign until after the merge has taken place and may in fact drop out.
I asked my solicitor if my house was therefore unsaleable and they said not at all; the issue was with the buyer who was being unreasonable.
I am obviously very worried; not just about the buyer not signing but about what happens next if they pull out. I need to know the following:

Is this a common situation and how can I resolve it?
Why can the solicitor not be certain the merge will happen after completion?
Is there anything I can do (contact MP etc.) to get the Land Registry to allow me to do the merge myself?
Can it be really possible I've bought a house outright but will never be able to sell it if the Land Registry won't merge the titles and so nobody will ever buy it? 

Hi - it's not an unusual situation
A conveyancer can be 'uncertain' for a variety of reasons but often it's related to the needs/requirements of the buyer's mortgage lender 
You wouldn't involve an MP. If you wish to apply for merger yourself then you can - see PG 26 section 3 - Leases: determination (PG26) - GOV.UK (www.gov.uk)
A lot of properties are both freehold and leasehold. The lease may still be wanted so both can and are often sold together and as one property. So no, it's not possible to be unsellable but a buyer, their conveyancer and lender have every right to refuse to complete on a purchase. 

Thank you for the reply.
My solicitor has told me it would be unlikely that a merge would be successful if I applied for it myself.
Is this not correct?

C Macpherson - anyone can apply so whether it's you or a conveyancer matters not.

Sorry, I didn't explain myself properly. My solicitor (and her director) have said the merge would probably be refused by the Land Registry if either I or my solicitor applied for it as there is no clear "intent".
They have told me the merge needs to be done by the buyer but they will not sign contracts under these circumstances.

This is why I feel so stuck!

C Macpherson - as I posted simply by applying shows the 'intent'. An owner of both titles/tenures can apply at any time to merge them. 
If they mean something else then best to ask them to clarify. 

Thank you.
Can I just confirm I am talking to a representative of the Land Registry?
And also do you have any idea how long this might take if it were expedited?

C Macpherson - you are posting on the HMLR forum and I represent HMLR.
Expedition generally means it is considered within 2 weeks of being expedited. Everything then hinges on it being in order, starting with the requirements as set out on the linked PG 26 section 3 being met

Thank you so much.
I will now go back to my solicitor.