You can NOT get the Chad visa in Nguimi . Nguimi is a hell of a town , full of scammers , ripp-offs , a few shops and nothing else.
There is not a clear entry point for TChad . I stamped my passport in Daboua (Its not in the Michelin , but the tracks will lead you to it ) .They were serious about the Visa .. and would be really complicated not to have one.
.... mostly because the real customs procedure are done in Bol .. and for that they examine your passport , entry point , visa , photocopy it a few times , another picture , pay here and there .. and I don't believe they can give a Visa in Bol ...
so .. that leaves you going tio NJamena without a Visa which would be mostly foolish .. as military and police are really serious in TChad. Id go from Agadez to Niamey and then to Zinder ... etc/// Its only 1 day more .. -you see Niamey which is TOP! - and get the visa on the spot -- 15.000 cfas
your other option is faking it . Most african Visas now are hi-quality stamps or even stickers on your passport really difficult to forge. But the Tchad visa is just a ink stamp , so crudely done that It can be really easily forged . just order one of these "custom stamps" on the web with the visa-design and stamp your own passport .
Have a nice day