babarage, I do this semi-regularly with my Mum, who like you is a 'level 2', i.e. she can walk on and off the plane, but otherwise needs assistance.

A few comments:

Given the large distances at T5, walking them with crutches is not a good idea. Leave it to Omniserve to help you.

We always ask for her wheelchair to be returned to the aircraft steps (or more accurately the halfway house on the airbridge), which gives us more flexibility, but it does take a bit longer as you need the wheelchair to be taken up from the hold. If you are not doing that then normally the Omniserve person is already there at the gate with one of their own chairs. Keep in the mind that often you will be asked by the cabin crew to stay seated until all the other pax have disembarked.

Omniserve have been at the gate every time, never any No-shows or late-shows. However I have noticed a few times that once they get you installed in the chair, they ask me "can you take it from here?". I have always said no - I am carrying hand luggage and my Mum's sticks, so can't handle a chair as well. Plus Onmiserve will guide you to the front of queues and they'll help expedite you through security by taking you to the front of the line. I also noticed that they have an app that tells them the probable next departure gate - and this seems to be in advance of what you can see on the BA and Heathrow apps.

It would be difficult to tell in advance which gates you will be using, but your inbound is more likely to be at an A-gate and the Boston outbound more likely to a B- or C-gate. So you are likely to be going pretty much doing arrival > security > outbound in quick succession. I think 90 minutes is doable - just keep the Ominserev person with you at all times!

If you can use a lounge (although your connection time might mitigate against that), Omniserve would leave you there and pick you up again. This is where they sometimes fail, as their timeliness for the pick-up is often poor.

Good luck and Happy Trails!